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Some questions and doubts about new CO2 setup

amateurfishkeeper

amateurfishkeeper

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30 May 2015
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I'm not a chemist or botanist so trying to understand the ideas behind use of CO2 in tank and how to find the sweet spot where things work well.

I'm confused by the word diffusion. I have a 'diffuser' which creates a mist of fine bubbles and I'm trying to find a way to create good flow in the tank. Some of the bubbles are blown on a merry journey around the back glass of the tank at a low level by a Hydor wave and circulation pump. Another pump on the other side of the tank creates flow across the front of the tank. With this kind of flow the water in the tank seems full of the tiny bubbles etc. I'm dosing the tank using the EI method etc.

Does diffusion mean the bubbles have actually disappeared or is it the creation of the bubbles? For example. if a tiny bubble makes its way to the surface of the water and breaks does that mean no CO2 was added to the water by the bubble? Has that bubble not diffused. Similarly if loads of the bubbles which escape the flow are sitting attached to the underside of the spray bar for ages is that a good thing. People seem fond of saying 'keep the bubbles in the tank for as long as possible'. Is that what they mean?

So I'm confused about what diffusion actually means and how it relates to the plants. Do the bubbles have to directly hit plant leaves for example? Or are the plants extracting CO2 that has 'diffused' into the water?

Since I've started adding CO2 I've had problems that didn't exist before. Green spot algae has appeared (always used an 8-hour photo period) and some plants that were doing well before have started dying off. Does CO2 kill some plants when added in this way?

So would like to be more clear about what diffusion really means in this context. Is it the creation of the bubbles or is it those bubbles disappearing into the water under the water surface?
 
One of the science boys (tom. Darrel, clive) can give you a better answer re: bubble diffusion then I can so I'll leave that to them. But in regard to new problems happening when the CO2 is added there are a few reasons for the things you mentioned. The green spot algae could be an indication of low phosphates. Since you plants are no longer potentially being limited by lack of CO2 your nutrient uptake will have increased. Green spot algae, although a bit is common, is often more common when phosphate levels are below a threshold (i think 2ppm).

You plants loosing leaves is probably due to them adjusting to this new CO2 rich environment. The plant "sets up" the leave composition to optimise the light and CO2 load of the water. As the water changes to richer in CO2 the plant may choose to discard the older (set up to try and maximise the lower CO2 levels) leaves in favour of leaves that maximise the light. Similarly when you have new plants from a nursery they are often grown semi immersed to maximise growth with atmospheric CO2 when you submerge them their leaves often die back as the plant makes new ones for the new environ.

So in regards to your plants, just snip away older leaves that are looking bad and enjoy your new, turbo charged, CO2 aided growth. :)

Best Regards,
John
 
Diffusion is how much CO2 is absorbed into the water. CO2 bubbles eventually disappear the longer they are in water. You'll never get 100% diffusion with a glass diffuser (the bubbles wont disappear completely). Reactors can achieve this and inline diffusers come pretty close too, this is why they are more efficient.

Bubbles don't have to hit the leaves for the plants to absorb CO2, it's absorbed from the water. The bubbles just give you a good indication that the flow of water around the plants is sufficient.

With regards to your plants dying and algae, if you haven't touched your lights then I'm not sure. Are you dosing any fertiliser?
 
Yes Julian , I'm following the EI program carefully - well the recommended version of it anyway. Alternate Micro, Macro - a rest day - water change and then Macro on the water change day.
 
Are you using a drop checker? Hows your pH profile? Sounds like all your problems are CO2 related. I'd check for leaks using soapy water (get a sponge with some washing up liquid and just squeeze it over all the joints). If you're confident that there's no leaks, then it's either not enough CO2/inadequate flow combined with too much light.
 
Hi Julian thanks. Faily sure there's no leaks after checking.

Yes, I have a drop checker. Initially I tried to place the diffuser under the inlet pipe of my Eheim filter - all the bubbles went up into that and were sent back into the tank. I noticed though that this started to corrode the rubber seals on the spray bar so I stopped doing that. This method was causing good distribution of CO2 I guess because CO2 levels were very rapidly rising. So instead now I'm relying on creating a lot of flow in the tank. I have two Hydor wave and circulation pumps. One is angled downwards at the diffused and aimer at the front glass. This causes the bubbles to clearly race across the front of the tank. The second pump at the other end of the tank picks those bubbles up and sends them round the back of the tank. The water looks saturated with CO2 bubbles - all the plants are swaying everywhere in the tank. I've got surface disturbance down to just a ripple effect at the top of the tank. This way of doing things results in the CO2 taking much longer to register. I mean before the drop checker would get from blue to green to dangerously yellowish levels rapidly. Now it takes much longer. It takes the whole day for the drop checker to go from blue to a greenish tinged blue. I'm having the CO2 come on three hours before lights on in an effort to create the build up of CO2 I imagine I need. My bubble rate looks fairly rapid - around 3 per second.

When there was a lot of CO2 in the tank my PH (according to pen) was 6.9. At other times of the day it was around 7.2 or 7.3. That's something that confuses me. People talk about a 1 unit PH drop meaning you have approximately 30ppm of CO2 in the tank (if that's correct - not sure). Is that the only way you could have 30ppm. Is it possible you can have that level without a 1 unit drop? If my with CO2 reading is 6.9 it seems strange - I've never seen my Ph at 7.9. I'm cautious about the amount of CO2 I'm adding because of the initial effect it had on the stock - lost a few. So I moderated the amount. I gather this is about finding a 'sweet spot' in the tank and the advice I get from here is tremendously useful. It's perhaps starting to look as if the missing part of the light, nutrients, CO2 equation could be the intensity of the light. I have two bog-standard Interpet screw in 18W bulbs. I'll go on with this approach for a while and then see if the light could be the missing piece of the puzzle,
 
5678 said:
my understanding is that water hardness affects the ph drop for a set amount of co2
Click to expand...
Nope. About 30ppm CO2 will always produce a pH drop of 1 regardless of water hardness, which is why you are looking for a change rather than an absolute value.
 
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