30-40ppm of CO2 harmful to fish?


Junior Poster
Mar 5, 2007
I have read a number of articles that says adding any more that 15-20ppm of co2 is harmful to your fish. I have 130watts of light over a 20 gallon tank. Ive kept my co2 at 20ppm's so i dont kill of my fish. yet for better results a number of people have told me with that high of light i should have it at 30-40ppm of co2. Any comments on this issue would be apprieciated?

Tom Barr

Staff member
Jan 23, 2005
Well, you could say 15 ppm is bad too.
Or any added CO2.

Are fish okay over the long term at 30ppm?
I've been doing this for 16 years.
My fish live a long time and breed, never get diseases.

There is nothing I can reasonably attribute to ill effects/impacts/health/reproduction/behavior etc.

What specifically is bad about 30ppm of CO2 in conjunction with good plant growth?

I can tell you why and where folks get the wrong idea on this issue.
1# Poor circulation (poor mixing of CO2/not enough flow through the reactor etc)
2.# No surface movement or not enough(low O2, no way for build up CO2 to escape, you lose some cO2, but it is easy to add more, turn the knob a little farther to the left etc).
3# poor plant growth(drains O2, does not add O2)

Many folks mistake CO2 issues for O2 issues, if you have high CO2 and O2 at the same time, things are fine. Fish experience that everyday in densely planted ponds/lakes/etc.

We should only add CO2 during the day, at night it should be shut off.
At night, after 1 hour or so the O2 ppm's drop after plants stop photosynthesizing.

It(O2) should not drop below 90% by morning when the lights come back on.
You need more circulation if so.

It's the combination that's critical in other words, the CO2/O2 high combination during the day and the low CO2/O2 combination at night.

If you have poor O2, that's deadly. Fish come first there.
You do not reduce current a great deal in order to conserve CO2.
Bad idea.
What happens at night and how you exchange nutrients/CO2 effectively without good current?

Low CO2 is no big deal, you add more, trim the plants and the algae.
You slowly add more, you can use a pH/KH drop checker with a KH reference solution, that's fairly accurate and accounts for KH tank water related issues.

You can also see the water report and ask how much bicarbonate alkalinity they have. Then use a ref and see if it matches.

If so, then use a pH meter, this will give you more accuracy, using the pH meter + KH reference will give even better results.

You can also do massive water changes a few times and then take a tank water KH. I think you can get a feel for the KH and pH pretty well using any one of these and the KH ref will add a lot more accuracy for you and give a nice test range for the test kit if you use one(none required if you use the KH test reference solution).

Most test kits and methods will only over estimate the CO2, not under estimate.
Folks have been adding 30ppm for a long time, A German fellow from at least 20 years ago suggested 30-40ppm according to Karen Randall. I just upped because it worked and my fish have always been in excellent shape.

I tend not to listen to hypothesis that do not match with what I see by all reasonable accounts. Some folks like to call those myths.

Personally, I like to see why the myth developed and what about it might lead folks to believe it. I try to figure out why it might work well for one person, but not another person.

Hope this helped,
Tom Barr