Basic Understanding of Dissolved Oxygen in Freshwater

[Bradders]

Hi All,

This is basic by some of the experts on UKAPS standards, but I did find it a compelling introduction to dissolved oxygen, which even I could understand! 😀

https://freshwater-aquaculture.extension.org/wp-content/uploads/2019/08/Dissolved_Oxygen_for_Fish_Production.pdf

It's not written for indoor aquariums, but I think this is a good general reading for Aquarists. There is an excellent section on predicting oxygen levels overnight while you sleep.

I have access to a dissolved oxygen meter now and am looking to do some experiments. Why? Because I love technology and experiments! 😱

Regards,
Brad

 

[dw1305]

Hi all,

..... but I think this is a good general reading for Aquarists.

I think so, too.

I have access to a dissolved oxygen meter now

<"What type?"> The only reason I don't recommend dissolved oxygen meters is <"just the cost">. Not everyone is a fan however and I've never done any real experimentation with them. I used to have <"incredibly polluted water"> to work with, and now I mainly <"work with pretty clean water">.

..... Just recently we tested a myriad of DO meters in the lab and the results were: they suck.

All kidding aside, the only reliable meter we have found and continue to use is a Hach meter, but these are > $2,000 USD. When I say reliable, I don't necessarily mean only accurate, but also precise. We've found that the meters require constant recalibration, and even with constant recalibration, their readings never seem stable or replicable ........

and am looking to do some experiments

This should give you some <"ball park figures"> and the quick and dirty temperature - %saturation - mg/L conversion chart.

cheers Darrel

 

[Bradders]

<"What type?"> The only reason I don't recommend dissolved oxygen meters is <"just the cost">. Not everyone is a fan however and I've never done any real experimentation with them. I used to have <"incredibly polluted water"> to work with, and now I mainly <"work with pretty clean water">.

Yes, well $2,000 is well out of my price range! (However, if I had that money you can bet I would buy one!!). The one I got was cheap (below) and I think at best it can be use as an "approximate" and "relative" testing tool. But I bet that the accuracy of these things for < £200 is no where near lab grade.

https://www.amazon.co.uk/dp/B0CC5HDW2J

 

[Bradders]

In terms of 'type' it the one that has electrolyte solution and you have to wave around in the aquarium to get a reading.

 

[dw1305]

Hi all,

But I bet that the accuracy of these things for < £200 is no where near lab grade.

That honestly <"looks OK">.

and you have to wave around in the aquarium to get a reading.

Calibration is just in 100% moisture saturated air, so basically you take a sample of RO water, put it in a beaker (where the probe is a fit), give the water a good shake and then place the DO meter in the oxygen (and water vapour) saturated air above the water surface and calibrate to 100%.



Once you've calibrated the meter you dip it in the tank water, give it a swirl and the dissolved oxygen diffuses through the gas permeable membrane to the sensor and gives you a DO reading. It looks like it has Automatic Temperature Compensation (ATC), but you can always use the chart.

cheers Darrel

 

[Bradders]

Calibration is just in 100% moisture saturated air, so basically you take a sample of RO water, put it in a beaker (where the probe is a fit), give the water a good shake and then place the DO meter in the oxygen saturated air above the water surface and calibrate to 100%.

Thanks Darrel. In the manual, it says to calibrate by leaving in the air. Do you think that is wrong?

 

[dw1305]

Hi all,

In the manual, it says to calibrate by leaving in the air.

OK, I've never known a meter like that, but it might depend on the nature of the sensor.

Do you think that is wrong?

I wouldn't have thought so, but you can always check with some aerated RO water. The 100% saturation value should be

..... for an aquarium at ~27oC and a barometric pressure of ~1015mb, will have a dissolved oxygen of about 8 mg/l,

cheers Darrel

 

[Bradders]

I have my new aquarium and am just getting it right at the moment. I had a few issues with floating wood, so the many plants there are just waiting for me to fix the aesthetics after they got uprooted! Replanted and reattached in a mess for now.

VERY rudimentary testing at this point. I am running a Lilly pipe on a 250L aquarium, which does not cause a huge amount of surface agitation, but there is some. At 08:30 this morning, the DO was ~4.8Mg/l. This is with no fish stocking at all. So the plants were no-doubt consuming oxygen overnight, and (depending on where you read the research) below 5mg/l starts the stressors on aquatic life. I am going to perform another test during the photosynthesis period to see how the levels change. (Ill do a few at noted times). Once the rudimentary testing is out the way (first pass) I can get a little more scientific. (Well, for me, anyway!)

I do see a lot of planted aquariums with Lilly pipes or gentle laminar flow only - and it makes me wonder about DO in the early hours of the morning (outside the photo period) in a heavily planted aquarium.

 

[dw1305]

Hi all,

VERY rudimentary testing at this point. I am running a Lilly pipe on a 250L aquarium, which does not cause a huge amount of surface agitation, but there is some. At 08:30 this morning, the DO was ~4.8Mg/l.

That would concern me a bit especially combined with your CO2 drop checker <"not fully degassing overnight">.

So the plants were no-doubt consuming oxygen overnight

They do, but they are <"massively net oxygen producers">, so without the oxygen from photosynthesis things are actually likely to have been worse.

, and (depending on where you read the research) below 5mg/l starts the stressors on aquatic life.

It depends a little bit on the <"fish you want to keep">, but personally I would want to ensure higher dissolved oxygen levels.

I am going to perform another test during the photosynthesis period to see how the levels change. (Ill do a few at noted times).

It should go to 100%, <"so about 8 mg / l">. I'm going to guess that if it doesn't? You need clean / remove / change some of you filter media.

I do see a lot of planted aquariums with Lilly pipes or gentle laminar flow only - and it makes me wonder about DO in the early hours of the morning (outside the photo period) in a heavily planted aquarium.

An <"air stone works wonders">. In planted aquariums fish death, associated with low oxygen levels, <"will always happen at night">, but what people don't appreciate is that the plants have kept their fish alive until then, and that without plants it is always "night".

Cheers Darrel

 

[Bradders]

Hi all,

That would concern me a bit especially combined with your CO2 drop checker not fully degassing overnight.

They do, but they are massively net oxygen producers, so without the oxygen from photosynthesis things are likely to have been worse.

It did concern me somewhat, too. I was expecting 5mg/l plus - so to get under that value WITH lots of plants AND CO2 injection AND no stocking was very odd. This is a new tank with new filters and fresh water. I need to look at this more closely.

It should go to 100%, so about 8 mg / l. I'm going to guess that if it doesn't you need clean / remove / change some of you filter media.

My photo period is currently set from 14:00 to 20:00. I'll get a reading at the start, the middle (17:00), and the end to see where we are.

 

[Bradders]

Update so far on the readings. (17:00 was late, sorry).

08:30 - 4.80 mg/l
14:00 - 5.34 mg/l [Lights on]
18:00 - 6.25 mg/l
20:00 - TBC

 

[Bradders]

Due to dinner constraints, I had to finish early, but you can see the increase throughout the photoperiod. (Worth noting - if relevant - I am currently only running lights for 6 hours and 25%). The lights turn off at 20:00, so I guess the question now is what will the DO level be at 08:00 tomorrow? Chances are it's going to be around the 08:30 figure. I wonder if it would be higher when I have the lights on for 8 hours and closer to 50%?

08:30 - 4.80 mg/l
14:00 - 5.34 mg/l [Lights on]
18:00 - 6.25 mg/l
19:30 - 6.51 mg/l

 

[Bradders]

So, the exact (first pass, non-stringent) process is summarised below. The DO probe was re-calibrated before every test using the same method.

With a slight adjustment to surface agitation via the lily pipe, you can see a healthier start to the day regarding DO - a 26% increase.

Sun 08:30 - 4.80 mg/l [Morning reading when minimal surface agitation occurred overnight]
Sun 09:00 - 4.80 mg/l [Upped Lilly pipe closer to the surface to create more surface agitation]
Sun 14:00 - 5.34 mg/l [lights on]
Sun 18:00 - 6.25 mg/l
Sun 19:30 - 6.51 mg/l [Just before lights off at 20:00]
Mon 08:30 - 6.07 mg/l

 

[nijat11]

Regarding this topic. I have a lot of surface movement in my aquarium (6500l/h), but recently I added two big air stones in my sump and they turn on 1 hour before lights go off (CO2 was off by that time) and it runs during the night till 9 AM. I think I can see more positive effect on fish they became more active and maybe colors are better. I dont know if there any effect on plants or not, because I did not big problems with plant.

 

[Bradders]

Regarding this topic. I have a lot of surface movement in my aquarium (6500l/h), but recently I added two big air stones in my sump and they turn on 1 hour before lights go off (CO2 was off by that time) and it runs during the night till 9 AM. I think I can see more positive effect on fish they became more active and maybe colors are better. I dont know if there any effect on plants or not, because I did not big problems with plant.

I did run a test using an airstone in my 100L aquarium. Let it run for a few hours, and my last measurement was around 7.6mg/l Dissolved Oxygen. They certainly do work!

 

[dw1305]

Hi all,

(Worth noting - if relevant - I am currently only running lights for 6 hours and 25%).

That is very relevant, along <"with it being a tall tank"*> (I assume it is this tank?). The tank dimensions, with the 50 cm height, means that you have a relatively small surface area to volume ratio and the smaller gas exchange surface area, unsurprisingly, reduces gas exchange. If you increase the photoperiod it should mean that you get to oxygen saturation before "lights out". You could actually use the DO meter to adjust lighting intensity and photoperiod to ensure full oxygenation.

Personally I don't actually ever fill my tanks up with water, I like some room for <"emergent growth">. This is @Iain Sutherland's lovely "Copella tank", and you don't need to have that little water, but it illustrates what you can do. Because I use rainwater a chalky "tide line" isn't an issue for me.

and this one @BigTom 's <"Tom's Poco Pozo">.

did run a test using an airstone in my 100L aquarium. Let it run for a few hours, and my last measurement was around 7.6mg/l Dissolved Oxygen. They certainly do work!

but recently I added two big air stones in my sump and they turn on 1 hour before lights go off (CO2 was off by that time) and it runs during the night till 9 AM. I think I can see more positive effect on fish they became more active and maybe colors are better.

It is a KISS solution.

cheers Darrel

 

[Bradders]

That is very relevant, along <"with it being a tall tank"*> (I assume it is this tank?). The tank dimensions, with the 50 cm height, means that you have a relatively small surface area to volume ratio and the smaller gas exchange surface area, unsurprisingly, reduces gas exchange. If you increase the photoperiod it should mean that you get to oxygen saturation before "lights out". You could actually use the DO meter to adjust lighting intensity and photoperiod to ensure full oxygenation.

Indeed. Once everything is bedded in, then it will be interesting to see if it gets to full saturation. More importantly, how that carries forward over night and next morning.

 

[dw1305]

Hi all,

then it will be interesting to see if it gets to full saturation

I would adjust the light intensity and duration until you get to ~8 mg / L dissolved oxygen (DO) towards the end of the photoperiod.

If you don't add CO2 you may find that the pH will rise rapidly as photosynthesis depletes the available CO2. I don't supplement CO2, so I use <"water movement"> to increase gas exchange for both CO2 and oxygen.

.... I've been surveying, and doing some water quality work, on some of <"Bristol's pond and rivers"> over the last couple of weeks.

I had an interesting one today. I won't tell you the location, or context, but it was a pond and the water sample had a dissolved oxygen level of 180% (~20oC, 18mg/L DO) and a pH value of pH 10.5.......

In low tech tanks water movement offers you a negative feedback loop, because the rate of gas loss (or gain) is dependent upon the concentration gradient, between tank water and atmosphere, at the gas exchange surface.

cheers Darrel

 

[LMuhlen]

I would also not worry too much considering it is a new tank being cycled. Who knows what those pesky microbes are up to? They may change their oxygen consumption pattern daily for all I know.

 

[hax47]

That would concern me a bit especially combined with your CO2 drop checker <"not fully degassing overnight">.

I think this is an important point to note. Low O2 levels combined with CO2 not escaping during the night suggest that gas exchange is very slow, most likely due to low surface agitation. Increasing surface agitation would help with both oxygen delivery and CO2 degassing during the night. While increased agitation would require dosing more CO2, it would likely also lead to reaching the target CO2 level more quickly during the photoperiod.