Balancing c02


Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida
Hi Tom,

Couple of questions on 'enough' c02 based on observation of tank conditions.

Observation: My plants pearl well and I get good mist, nice growth, and no algae.

Observation: I notice my school of cards (250) are more active within 10 minutes of c02 going off and they hide more later in the day when c02 has been on a while. I did lose one or two recently, but unsure if c02 was the cause.

I reduced the c02 anyway.

Observation: After a water change, I get more pearling than normal but not like a whole lot more, but noticeable.

1. Does the fact that I get more pearling (from the HC mostly) indicate I am still short on 'optimal' co2?

2. Does the fish behavior indicate that I have reached their comfort level in terms of c02?

3. Will the plants not do 'optimally' if I must keep c02 lower than 'needed' to keep the fish healthy and happy?

Plant growth is great, leaf shape nice, color is coming in well. Better growth closed to surface of course, but all areas of tank are doing well.

No new algae, old almost all gone, etc.

4. Will a tank always have 'some' algae present no matter how well balanced? I am not looking to 'eradicate' algae, but if plants are doing well and c02 is stable and sufficient, I would think that there is minimal algae present.

Your own tanks are proof of that.........

5. With algae almost gone in my tank, is there a 'need' for algae eaters, or will they starve if I reach
'planted tank nirvana' when all is optimal? I would like to have some ottos again and also try some RCS, but don't want them to starve.

6. Can I feel that the 'best' c02 I can get is where it is based on plant health and fish behavior?

Better post water change pearling indicates co2 could be better and fish tell me I am borderline too much.

Any tips to help balance the two goals? I love my fish more than my plants......

As always any thoughts are appreciated.

Misc question:

1. I have a 180 and between 250-300 cards, a dozen espei and that's it. What is a MAX stocking level for both species in the tank? I would like to have 50 MORE espei eventually if I can get them. I have good mech and bio filtration with the NC canisters.

Tank looks 'empty' at times :)



Guru Class Expert
Feb 17, 2007
Lincoln, UK
I think any observations of pearling after water changes should be taken in the context of the situation.

I would always expect higher pearling on all plants after a water change because the water should have more O in it that the tank water (assuming Tap, pressurised) and therefore less O has to be produced by the plants to reach saturation point where the O forms as bubbles because they can no longer go into solution.

However on the CO2 front we use the 30ppm as a target. In general as per Tom's findings at the ppm at the CO2 source may be 100ppm+ and further at the substrate 10ppm while the DC suggests 30ppm. Its the average we are looking for.

If we assume that the substrate is 10ppm and the plants grow well at this level then what does this tell us? It says that if it were possible to maintain the 10ppm throughout the tank with no variance that we would get the same results as we currently get with the huge variances. The 30ppm average is basically maintaining the 10ppm level in the poorer areas while giving excess in other areas.

It also shows that 30ppm is not the safe level but more the safe average otherwise when the fish went near the CO2 source (my Rasboras play around the diffuser almost all of the day!!!) they would hit the 100ppm+ area and rapidly move away which they don't.

What I am getting at is in a tank with much much better circulation (and I mean good) then you can get away with injecting less because the 10ppm/low end of the measurements would still be maintained. In a tank with poorer circulation you would need to inject more to keep the low end at a decent level whilst the top end (source) could now be much higher than the 100ppm. The average may still be 30ppm though.

Another observation I would add (I have 17.6x volume turnover and very good circulation) is that with a ceramic disc the bubbles come out as mist for half of day1 and gradually get larger through the week. The DC (I use this as guide and not as gospel) gradually gets darker towards the end ofthe week so starts the week at lime green which I assume 35ppm avg. and ends the week at a darker green which I assume 15-20ppm. The plants don't get algae though. I make the assumption that my circulation is maintaining the lower level of CO2 and therefore I am not needing to reach the higher level to maintain this level.

I could speculate that the equilibrium (which may or may not be 8ppm depending on which articles you read) is similar to this lower level and that the plants max out of need well before 30ppm meaning that as long as they get say 15ppm (example) then anymore does not mean more growth. This is pure speculation though. Someone is bound to suggest that this cannot be true otherwise why would plants suddenly grow much faster when they are emersed. the answer for this would be easy (as I see it) in that the plant chnges when grown emersed and maybe it changes to make use of the higher amount of CO2.

I would go so far as to say if people would listen to the lower light suggestions rather than wanting to have floodlights that they could get away with using half as much CO2 and use 2dKH solution in their DC to aim for green. This would still mean great circulation needed.

As for Otos and Shrimps they will only starve if you don't feed. Shrimp are scavengers and will eat anything that hits the bottom. If you get some Catfish sinking pellets you will find the Otos and Shrimp will both go for it.


Tom Barr

Staff member
Jan 23, 2005
The same argument exist with using less light, then you need less CO2........and less circulation.

But then you have less fish also...............and lesser plant growth(not a bad thing really)

Tom Barr