EI co2 and fish health

Henry Hatch

Guru Class Expert
Aug 31, 2006
I've been using EI and co2 for about 3 years and have had more success with this approach than any other.

However, I have been nagged by a question for about a year regarding the issue of fish health.

I kept fish for about 30 years in non planted tanks before I got involved with planted tanks.

It has been my observation in my battles with algae, particularly BBA, that in order to keep the BBA at bay I need to bring co2 levels to the edge. By the edge I mean a level of co2 just below where fish show obvious signs of stress.

My concern is that with the co2 levels one normally uses with EI that fish which might not show obvious signs of stress may be suffering from low levels of stress which may be hard to see.

It seems to me that when I kept fish in non planted tanks that the fish seemed more active. I keep many of the same fish in my planted tanks now that I kept in my non planted tanks. Obviously there are many things which can affect the activity level of fish other than co2.

This may be nonsense on my part, but it is something which has crossed my mind.



Lifetime Charter Member
Lifetime Member
Jun 21, 2009
Surprise, AZ
Not Nonsence At All

I don't think it is nonsense at all, I am not sure we can say with any certainty what effect relatively high doses CO2 or the other interactions that may be taking place over time are.

Most of the information I am familiar with though relates to the transport of fish for food production.

Having said that, I wonder about circulation efficiencies (deficiencies). The point at which most critters run into problems is rather high at least in my experience.


Tom Barr

Staff member
Jan 23, 2005

With lower light intensities, there's less need to approach such a fine edge.
Good high rates of circulation and O2 are factors also, and allow a wider range of CO2 to added without ill effects on fish or behavior.
I've bred Angels consistently, at 40ppm of CO2, Discus, Cories, maybe a few fish I'm not even aware of.

If they are breeding, they have good behavior, that's fairly safe to say.

How you approach the addition and adjustment of CO2 varies greatly person to person. Some are rather quick and rash, some assume you just add it and that's all there is to it. Some are scared, adding barely enough and adjusting it only a little.

Some change their tanks and the water level which drives off more CO2, some raise the water level or the biomass reduces flow which conserves CO2.
Evaporation can cause a lot of that.

Fish species seems to play a little role also.
Certain species of Plecos are more sensitive than most other species I've kept.
Tetras are tough as nails. Still, these things all play roles, so less light, better mixing and consistency for plant biomass, water level etc will aid in the reduction of the BBA etc, some use Excel instead.
Some go non CO2.

There are ways to make it easier to hit without stressing fish and reducing their stress levels.

If you feel strongly about this, you will want to crusade strongly against using pH controllers, CO2 24/7 etc.........ask folks to use lower light, more current etc.

You'll notice I do as a matter of philosophy;)
As well as non CO2 gas methods as well.

Tom Barr