# Co2 2hr rule myth



## Danny (7 Feb 2018)

Anytime someone asks when the co2 should come on either on forums or facebook etc  the answer is pretty much generic at 2hrs before lights on, everything you read about it in Google also says the same.

Being totally new I followed that blindly as I'm sure everyone does, the thought of the affect of natural light didn't even cross my mind.

However, even the smallest bit of natural light will have an affect as I've noticed.

I would sit and look at the plants reaching over towards the natural light without even a second thought of having the co2 on as my lights were not on, then once the lights came on later in the day the plants would correct themselves and straighten up towards the light.

After a lot of thinking and confusion about algae issues I came to the conclusion that in fact although I thought my co2 distribution was fine I was I was under supplying the tank by 6hrs based on the affect of the natural light.

My lights come on at 3pm, the co2 was at 1pm the standard 2hrs before lights on. The natural light takes affect on the tank anywhere from 8am on some days and the plants will start reaching towards it.

Why is the default answer about when the co2 should come on 2hrs before lights on? I followed it blindly and I am sure caused myself massive issues due to under supplying the tank.

I wonder how many people with algae issues are also experiencing the same because of also blindly following the 2hr rule, I did see a couple of days ago a post by Clive which mentioned the same and confirmed my thoughts which helped me decide to adjust my co2 on time away from the standard 2hrs before lights on.

Does this make any sense to anyone lol


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## zozo (7 Feb 2018)

Danny said:


> Why is the default answer about when the co2 should come on 2hrs before lights on?



Because in general consensus for as long as the hobby excists with artificial light, the rule of thumb in any aquarium guide regarding aquarium location is by default to place it as far away as possible from any natural light source. So this default answer is given in the assumption you took notice of and the basic rules into consideration. These rules are not written in stone there are many other possibilities that work as well but require a different approach. The default answers in a basic guide does not cover every alternative, they only treat the for so far known best practice. 

I got a open top low tech aquarium standing straight under a skylight, facing a window recieving full blast daylight from 3 directions, from above, the front and right side. Theoreticaly the worst place imaginable  And to make it work i need to switch off the artificial light above it from march till october. There isn't an article to find anywhere in the world that told me so. All say it doesn't work, but it does only without artificial light. Had to find that out myself, can't blame the Basic Aquarium keepers guide for it..


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## Konsa (7 Feb 2018)

Hi
The 2 hours  rule is just a guide and is not to be followed blindly.
Depending on your tank size and injection metod you should adjust your CO2 start time to be able to get 1 point ph drop from your CO2 start time to the time when the lights come on.It may take anything from an hour to 5 hours and more to achieve that if your injection and distribution metod is not very effective.This will be the most accurate way to determine  CO2 levels as drop checkers have quite a bit of delay in registering changes.
The basic ph meter device cost next to nothing and is a good idea to have one.
Regards Konsa


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## Danny (7 Feb 2018)

I'm meaning in more simple terms of the standard "2hrs before lights on", if the lights come on at 6pm then by the 2hr rule the co2 would be at 4pm blindly. 

As I say I'm totally new to co2 and just took the 2hr rule for what it is rather than factoring in anything else including natural light as I'm sure a lot of people new to co2 do, I'm more trying to explain that than try and blame anything for following that blindly as there just isn't anything that says any more than to have it on 2hrs prior to lights on and I'm wondering why.


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## Danny (7 Feb 2018)

Example,

Q. When should I run my co2 from?
A. 2hrs before lights come on.

That is pretty much all that is seen everywhere and I'm sure the reason a lot of people like myself have been running into algae issues that are simply caused by following that 2hr rule blindly.


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## Danny (7 Feb 2018)

What I'm saying is that to perhaps most people there is a misconception that 2hrs before lights on is purely the tank light, not factoring in natural or room lighting.

Unless you live in a cave the tank will get some form of natural or ambient room light, would it not be better to base the co2 start time on the time the house wakes up/natural light starts hitting the tank?

I have adjusted my own co2 to run from 7am now which is 8hrs before my tank lights come on rather than the generic 2hrs before.

I'm probably explaining my meaning wrong but hopefully it will make sense.


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## Danny (7 Feb 2018)

So this is my tank in natural light, yeah it is a lot but for one reason or another having the co2 on before the 2hrs before my lights come on just went straight over my head.

The plants would all reach over to the light and purely based on the 2hr rule it didn't even cross my mind that I should have the co2 on, perhaps stupidly I know but I was just following the "rule". By doing so I was basically starving? the tank of co2 for 6hrs before it came on as the natural light is enough to have the "lights on" affect on the plants to "wake up" and begin photosynthesising.

So I am thinking would it not be better to generalise that the co2 come on at say 8am for example than the current 2hrs before lights on? That 2hrs before lights on in my case was 1pm and in others could be even 6pm or later.


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## zozo (7 Feb 2018)

As said aquarium guides follow some basic rules under the assumption you use basic equipment. E.G. you have a light on a timer switch that switches on full blast at a given time. And switch off agian to absolute 0 light at the end of the programmed cycle.

Best practice is to make sure the water is saturated with enough CO² the moment the lights are switched on and stays at a stable level for the whole periode in this light cycle . During the night when lights are off, basicaly your CO² is off. During this dark periode the CO² contents gradualy lowers to a natural equilibrium. The moment your CO² installation starts pumping it into the tank it takes an amount of time to slowly get to te desired contents, basicly it takes about 2 hours. Meaning if you start adding it the moment the lights come full blast on than there will be to little CO² in the water the moment the plants start metabolizing. And thus resulting in a unstable CO² contents..

All this under the assumption everything else is simply basic, meaning timer switch on your light, timer switch on your co2 and the aquarium standing in a spot without natural light influences.

Obviously if any of these basic rules are not met, in this case for you it is not met with enough natural light to make the plants react to it. Than all rules change and don't apply anymore. Than it will be very hard to describe in words how it excactly is different from the basics. Than you have to use your own common sense and experience to puzzle out what's the best way to anticipate to this to get a stable CO² contents with influence from natural light.

Also the 30ppm that is written all over the place, isn't a rule written in stone.. There are people saying they are only adding 15ppm CO² and have equal good results.. Now i have to say maybe, since i can't know, but you have to try things. And since if you recieve a lot of natural light, than this will be also not a stable light source during the year.. You can't realy anticipate to this with a fixed on off schedule. Then try for example 15ppm CO² for 24 hours a day and don't switch it off at night.. That's what i would try if CO² is an absolute must have in natural light conditions.  Than at least it will be stable for the whole periode..


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## Danny (7 Feb 2018)

I think these days with the majority of homes being "light and airy" the general rules don't apply anymore, especially with so many homes now using led bulbs etc.

I have the lights and co2 on timers but directed all of my trouble shooting at the tank, the distribution and circulation when actually I'm certain my biggest problem was not factoring in natural and room lighting so starving the tank.

No matter how many ph profiles I would have made without factoring in outside light sources I would have never solved anything algae related that "shouldnt" occur with co2 when used correctly and I'm sure it is why a lot of people have problems too.


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## ian_m (7 Feb 2018)

See here fo rmy view on the maths behind CO2 concentration vs time.

https://www.ukaps.org/forum/threads/co2-concentration-versus-time-the-maths.51423/

So as a rough rule of thumb, with decent CO2 injection, acceptable CO2 surface loss and reasonable tank volume it appears that getting to 30ppm CO2 takes about two hours.

Also many people run their high tech aquariums in conservatories (not in direct sunlight) and don't suffer algea and/or other issues. So natural light is normally not an issue.

Found the link.
https://www.ukaps.org/forum/threads/forest-of-congo.23941/page-2#post-258486

My tank is by a window (handy for water disposal, pop hose pipe out of windows onto lawn) and doesn't have algae issues, other than caused by me mishandling CO2 levels and tank lights and excess feeding.


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## zozo (7 Feb 2018)

Personaly i'm a low tech nerd, that's how i learned it and did that for decades. And starting again after a break a few years back i red a lot about High tech and all articles stating with mythical proportions that CO² is the absolute Algae remedy. So i gave it a try.. Surprisingly i have 3 relative algae free low tech aqauriums, as said above one with a huge amount of natural light. And i never ever experienced such massive fast growing algae attacks as i did in my first and for now last High tech tank.  For so far the algae remedy. It took me a year and a lot of plantmass to get the hang of it in the 2 years i had it running.  I'm still not convinced if it was something i did or just the plantmass it fanaly grew making the algae go away. And this tank stands in the kitchen also receiving a small portion indirect natural light.   

The best discription of comparing High tech aquarium is still the "Crashing Race car".. Driving fast, than handling the steering wheel incorrectly and or little bumps in the road can have drastic consequences. And curing from the consequences always is more difficult and takes longer than getting it.


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## Danny (7 Feb 2018)

ian_m said:


> Also many people run their high tech aquariums in conservatories (not in direct sunlight) and don't suffer algea and/or other issues. So natural light is normally not an issue.



I don't think that light is an issue at all if the co2 is run correctly to match it as is said by those far more experienced, the fact that some super high light tanks are pretty spotless shows that.

In my case I am sure my algae issues were down to a lack of co2 for the lighting which is the commonly accepted reason for it, however my lighting issue I am certain was/is the natural and room light.

Because I dialled in my co2 to 2hrs before the tank light came on 1/3pm I was not providing co2 for the actual effective light on the tank which is the natural and ambient lighting which can be from 8am ish, basically the way I see it is that it is like having the tank light on from 8am but starting the co2 at 1pm. There was a 5hr deficit in co2 to light ratio purely based on my blind following of the 2hr rule.


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## zozo (7 Feb 2018)

@parotet i remember also had a CO² tank receiving a lot of natural light.. And he also scheduled his CO² starting earlier according too that with very good results. Can't find the topic back, it's from over a year ago if i remember correctly.. But he might be around and find this, to shed some light on his actual schedule he used on it.


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## Chubbs (7 Feb 2018)

There’s another post somewhere with a similar issue. I suggested basing it on when the plants open, rather than when your lights come on. As another poster has said that typically, you avoid having the tank in reach of natural light , because you loose ‘full’ control. I still stick with the 2 hours rule as it does seem to work, but just don’t base it solely on lighting, go with what nature is telling you.


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## Danny (7 Feb 2018)

Chubbs said:


> There’s another post somewhere with a similar issue. I suggested basing it on when the plants open, rather than when your lights come on. As another poster has said that typically, you avoid having the tank in reach of natural light , because you loose ‘full’ control. I still stick with the 2 hours rule as it does seem to work, but just don’t base it solely on lighting, go with what nature is telling you.


It could well have been your post I saw about basing it on the plants, I thought it was one of Clives. Which ever it was it definitely filled in the missing piece to my thoughts.


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## Chubbs (7 Feb 2018)

Danny said:


> It could well have been your post I saw about basing it on the plants, I thought it was one of Clives. Which ever it was it definitely filled in the missing piece to my thoughts.



I think other posters are infinitely more experienced I just share my own findings.
To me it makes sense to do it that way and it works great for me. Quite a lot of Plants don’t actually need much light to flourish and even ambient day light can be enough. Then add into the mix overly powerful LEDs, and it’s heaven for algae


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## Danny (7 Feb 2018)

Chubbs said:


> I think other posters are infinitely more experienced I just share my own findings.
> To me it makes sense to do it that way and it works great for me. Quite a lot of Plants don’t actually need much light to flourish and even ambient day light can be enough. Then add into the mix overly powerful LEDs, and it’s heaven for algae


From Rotala to AR mini they all reached for the natural light, if the daylight was the sole lighting say 9hrs ish now and no co2 was used then algae would be a nightmare so it makes sense that my 5hrs without co2 would cause algae too.


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## Danny (7 Feb 2018)

The 2hr rule to get to optimum levels still applies as good practice just not linked to the tank light but the external source in my mind.


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## Tim Harrison (7 Feb 2018)

Achieving lime green drop checker at lights on depends on several abiotic and biotic factors which usually, and luckily for us, remain fairly constant e.g tanks surface area, surface water movement, plant uptake (which increase steadily with growth) etc. So the 2hr rule is just a loose guide, it's not meant to be written in stone.

To accommodate all these variables my CO2 needs to come on 3hrs before lights on if I'm to achieve a lime green drop checker and maintain a stable CO2 conc throughout the entire photoperiod. The gas goes off 2 hrs before lights off.

It's difficult to compensate for natural light since it is often not consistant, and direct sunlight will absolutely cause algae.

This may also help https://www.ukaps.org/forum/threads/co2-2-hours-before-lights-turns-on-why.51367/


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## Danny (7 Feb 2018)

Tim Harrison said:


> Achieving lime green drop checker at lights on depends on several abiotic and biotic factors which usually, and luckily for us, remain fairly constant e.g tanks surface area, surface water movement, plant uptake (which increase steadily with growth) etc.
> 
> It's difficult to compensate for natural light since it is often not consistant, and direct sunlight will absolutely cause algae.
> 
> ...


But when "lights on" is accounted for it's purely aquarium lights.... I work from home a lot and have been able to spend hours looking at the tank with "lights off" and see just how everything reacts to natural light/ambient room lighting. Unfortunately it's taken weeks for something to click and get to this trail of thought.

I think "tank on" instead of "lights on" now, the tank is awake way before lights on, how many people go to work and leave the curtains open but don't have the lights or co2 on until say 5pm when they get back while the tank has been "on" all day with the ambient lighting......


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## Danny (7 Feb 2018)

I planted this cutting of Rotala rotundifolia 'Orange Juice' last night, it's been reaching for the natural light all day.

By 5pm ish it will be upright again as my light comes on at 3pn, on my previous schedule the co2 would have come on at 1pm but as of yesterday it's 7am.


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## Tim Harrison (7 Feb 2018)

Okay, I think I take your point...How many problems occur because folk are not aware that their tanks maybe getting too much direct sunlight or strong ambient light?
Rooms with windows have ambient light. But that's not usually a problem since it's usually not strong enough to exert an influence above and beyond that of the artificial light we use to grow plants.
If however, a tank is sitting in a south facing window then it's likely to get strong ambient light and direct sunlight, and that may cause problems.
Solution...don't place an aquarium in or near a window...unless it's one of these 
https://www.ukaps.org/forum/threads/the-banyan-forests-of-stickleback-island.12557/


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## Danny (7 Feb 2018)

Tim Harrison said:


> Okay, I think I take your point...How many problems occur because folk are not aware that their tanks maybe getting too much direct sunlight or strong ambient light?
> Rooms with windows have ambient light. But that's not usually a problem since it's usually not strong enough to exert an influence above and beyond that of the artificial light we use to grow plants.
> If however, a tank is sitting in a south facing window then it's likely to get strong ambient light and direct sunlight, and that may cause problems.
> Solution...don't place an aquarium in or near a window


I'm not great at explaining my trail of thought in ways others may understand lol 

But yes, no house is pitch black all day so the tank will get some form of ambient lighting be it natural or artificial.

It was looking into blacking out the tank that started this trail of thought and got me to look outside the tank for problems instead of going around in circles with the internals.


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## Tim Harrison (7 Feb 2018)

Sort of thinking outside the glass box


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## Danny (7 Feb 2018)

Tim Harrison said:


> Sort of thinking outside the glass box


Literally, 

I have been as sure as I can be that I'm doing everything as I should in the tank with co2 etc etc but nothing was getting any better with algae, yet the plants all grow like mad even buces were sprouting shoots all over the place.


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## a1Matt (7 Feb 2018)

zozo said:


> Also the 30ppm that is written all over the place, isn't a rule written in stone.. There are people saying they are only adding 15ppm CO² and have equal good results.. Now i have to say maybe, since i can't know, but you have to try things. And since if you recieve a lot of natural light, than this will be also not a stable light source during the year.. You can't realy anticipate to this with a fixed on off schedule. Then try for example 15ppm CO² for 24 hours a day and don't switch it off at night.. That's what i would try if CO² is an absolute must have in natural light conditions.  Than at least it will be stable for the whole periode..



I'd echo this.... try your co2 at a lower rate for 24/7. It sounds like it would work well for your setup. You might be surprised by how even a very low injection rate provides benefits (assuming you have distribution nailed).


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## Danny (7 Feb 2018)

a1Matt said:


> I'd echo this.... try your co2 at a lower rate for 24/7. It sounds like it would work well for your setup. You might be surprised by how even a very low injection rate provides benefits (assuming you have distribution nailed).


I did a brief ph profile and checked it against the ph/co2 chart which came back as off the scale lol 

Through trial and error I have the co2 as high as I can without the fish gasping at the surface so know I'm at my max level for it, distribution is in my mind great and diffusion is inline.

In terms of running it lower for 24hrs I would rather not as I do like to give the fish etc a break, plus at lights off the tank is in total darkness anyway.

I'm more focused on matching it to lighting now which I am doing based purely on the plants reaction to natural and ambient light rather than the tank light.

The tank wakes up at 8am ish so I'm running co2 from 7am, because I know I'm at the top end of injection I'm expecting the impact to be quick so starting 1hr before the tank wakes up and not 2hrs, also because I'm working purely based on my own assumption and not a given method as the standard 2hrs before lights on is.


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## a1Matt (7 Feb 2018)

That all sounds good, you've found an aporoach that works for you, so I totally aporeciate you not wanting to try 24/7. 

I'll address one point though, for the benefit of others who might consider it.





Danny said:


> In terms of running it lower for 24hrs I would rather not as I do like to give the fish etc a break, plus at lights off the tank is in total darkness anyway.



When advocating 24/7 my intention is low co2 throughout.  As it never gets high, the the fish don't need a break.


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## Danny (7 Feb 2018)

a1Matt said:


> That all sounds good, you've found an aporoach that works for you, so I totally aporeciate you not wanting to try 24/7.
> 
> I'll address one point though, for the benefit of others who might consider it.
> 
> When advocating 24/7 my intention is low co2 throughout.  As it never gets high, the the fish don't need a break.


The only benefit I can see of running 24/7 is having that added co2 when the tank wakes up which is where I feel the standard 2hrs before lights on ignored, there is no benefit of co2 over night so it can only be that the potentially ignored time is then covered.


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## Danny (7 Feb 2018)

Anyone that has their lights and co2 synced to say 5pm for example is potentially neglecting the need for co2 from when the tank wakes up, in my case 8am ish but will differ from room to room of course.


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## Edvet (7 Feb 2018)

a1Matt said:


> You might be surprised by how even a very low injection rate provides benefits


i had low 24/7 CO2 on this tank for about a year:


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## Danny (7 Feb 2018)

Edvet said:


> i had low 24/7 CO2 on this tank for about a year:


How long was your lighting period? Great tank


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## Danny (7 Feb 2018)

Lets work with an example of my theory based on summer time,

So lets say a room gets enough ambient light to "wake up the tank" from say 6am until say 10pm, 16hrs total

Then the tank light is run from say 4pm until 10pm, 6hrs

Co2 run from 2pm to 9pm, 7hrs

If the tank is awake and plants photosynthesising from 6am which is the desired affect we aim for by using tank lights right? then is there not a deficit of co2 between 6am and 4pm? 10hrs

If the tank wakes at 6am then based on the 2hr principle should the co2 not start at 4am?

Make sense?


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## Edvet (7 Feb 2018)

Danny said:


> f the tank wakes at 6am then based on the 2hr principle should the co2 not start at 4am?


It should , depending on ambient light levels, 16 hour daylight is just in midsummer though.
Basically if there is a lot af ambient light, the best thing to do ( in my mind) would be: start "CO2 at sunup ( stil low light, gradually increasing) 2 hours after sunup start tank lights, keep them on for 6 hours, then kill the tank lights and have the ambient light only. After those 6 hours the plants will stop the largest part of metabolizing (they "ate"their fill) and won't be hurt by the diminishing ambient light.


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## Danny (7 Feb 2018)

Edvet said:


> It should , depending on ambient light levels, 16 hour daylight is just in midsummer though.
> Basically if there is a lot af ambient light, the best thing to do ( in my mind) would be: start "CO2 at sunup ( stil low light, gradually increasing) 2 hours after sunup start tank lights, keep them on for 6 hours, then kill the tank lights and have the ambient light only. After those 6 hours the plants will stop the largest part of metabolizing (they "ate"their fill) and won't be hurt by the diminishing ambient light.


Awesome you have made sense of my theory lol


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## Zeus. (7 Feb 2018)

I have two tanks one takes 3hrs to get the pH drop whilst the other does it in 45mins. The one that takes 45mins a 500l tank does have duel CO2 reactors with a massive CO2 injection rate, once the pH drop is reached one of the CO2 injections is off then for the rest of the photoperiod, a 6Kg cylinder lasts about a month



The smaller 50l tank just has one atomizer and the 6kg CO2 cylinder has been on it about nine months


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## alto (7 Feb 2018)

I also run CO2 24/7 - despite tank not being adjacent to the window, sunlight directly hits the tank from sunrise to ~noon

I've never run pH profiles on my tanks, water is extremelily soft so I'd have to adjust tap water & I gave that up some while back  - low level 24/7 CO2 is dead simple & neither plants nor fish seem to complain 

Conversely I've never had much algae in this tank even with erratic CO2 & erratic photoperiod & at most 4x tank turnover/hour (flow distribution is decent)

If you want that high tech "ready in 8-12 weeks" aquascape that's not somewhere my lackadaisical approach will ever get you


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## Danny (7 Feb 2018)

Danny said:


> I planted this cutting of Rotala rotundifolia 'Orange Juice' last night, it's been reaching for the natural light all day.
> 
> By 5pm ish it will be upright again as my light comes on at 3pn, on my previous schedule the co2 would have come on at 1pm but as of yesterday it's 7am. View attachment 113150


Not sure how viable the difference is but I'll post another in the morning to show the affect of natural/ambient lighting.

This is only a 3" cutting though.


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## alto (7 Feb 2018)

Rotala rotundifolia varieties are the most flexible aquarium plants


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## doylecolmdoyle (8 Feb 2018)

I run 24/7 co2 in all my tanks, they all get natural light, one is even outside, they all do great with this method. All my fish and shrimp seem happy never had any problems with them gasping, 24/7 co2 at a low rate is worth a try if you have lots of natural light hitting the tanks.

I stand by this method, i have basically no algae in all tanks, even the tank outside (picture below, its way overgrown) has no real visible algae .



IMG_0847 by Colm Doyle, on Flickr


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## Danny (8 Feb 2018)

alto said:


> Rotala rotundifolia varieties are the most flexible aquarium plants


Good indicators for the affect of natural/ambient light then


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## Danny (8 Feb 2018)

doylecolmdoyle said:


> I run 24/7 co2 in all my tanks, they all get natural light, one is even outside, they all do great with this method. All my fish and shrimp seem happy never had any problems with them gasping, 24/7 co2 at a low rate is worth a try if you have lots of natural light hitting the tanks.
> 
> I stand by this method, i have basically no algae in all tanks, even the tank outside (picture below, its way overgrown) has no real visible algae .
> 
> ...


Why do you run co2 24/7? What do you think the benefits are and would happen if you didn't?


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## Danny (8 Feb 2018)

Danny said:


> Not sure how viable the difference is but I'll post another in the morning to show the affect of natural/ambient lighting.
> 
> This is only a 3" cutting though. View attachment 113153


This is early this morning, it's quite overcast today so let's see how it changes....


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## doylecolmdoyle (8 Feb 2018)

Danny said:


> Why do you run co2 24/7? What do you think the benefits are and would happen if you didn't?



The benefits to me are no need to worry about achieving optimal levels of co2 before lights on or before direct natural light starts hitting the tanks. Originally i started running 24/7 as my check valve broke and i noticed the fish didn't mind, the plants responded well and I stopped getting GSA and other forms of algae, I calculated i was actually using less co2 by running a low level 24/7 (say 1 bps) rather then cranking 4 or more bps for 3 hrs before lights on to achieve the 1ph point drop or 30ppm of co2. Now i use the same method on 5 tanks and they all run smoothly, dont need solenoids or timers etc. Need to make sure you have decent surface agitation and flow, for me I think I will always use this method until I see it isnt beneficial.

If I went back to using solenoids and timing the co2 to be at optimal levels before lights on... im sure everything would be pretty much the same, but this method works for me so ill stick with it.


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## Danny (8 Feb 2018)

Danny said:


> This is early this morning, it's quite overcast today so let's see how it changes....View attachment 113161


So here it is again, perhaps it is just a coincidence the AR mini is also reaching in the same direction as the natural light too lol 

If the natural/ambient light on a tank is strong enough to cause plants to reach for it I think it impossible to not factor it in as a light source and adjust co2 accordingly.


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