Too Much CO2?

Jeff Bodin

Junior Poster
Feb 19, 2005
Is it possible to have too much CO2 that it will inhibit plant growth?

I have a Carbonate Hardness that stays around 3.5-4 dKh and have noticed that when the pH is around 6.0 that growth almost comes to a hault (90-120 ppm CO2).

I've raised the dKh in some cases to get the pH higher without adjusting the CO2, but have seen no change in growth rate as a result.

I've got LOTS of light (4x55WPCFs) :cool: over my 55G (My fish may die of skin cancer ;) ) and figured the higher-light and higher CO2 combo would be beneficial...

However: the best growth rates seem to occur when I decrease the overal CO2 concentration, keeping the dKh constant (around 4) and allowing the pH to go up to 6.2-6.4 (still high per the CO2 charts) - this yeids a CO2 concentration of ~ 45ppm (still in the red zone).

I've done this (non-experimentally) several times and noticed the change in growth rates.

Any thoughts are appreciated.

- Jeff

Gill Man

Prolific Poster
Feb 10, 2005
San Francisco
Re: Too Much CO2?

You can only drive the photosynthetic reaction so far with light, CO2 and nutrients. After a certain point, adding more will not be of any more benefit and may even hinder or slow the reaction. Certainly plants come from waters that are even lower in pH values, yet grow and thrive in these waters. Obviously there in no CO2 reactor upstream, so there must be other factors involved in their success. You have found success in growth rates at a certain hardness level and reasonable CO2 level. I would try to maintain those conditions which have been shown to benefit your plants most. The CO2 charts are "theoretical" which is always going to be different than what you can measure or test due to variables in your tank. My theoretical CO2 level, at my KH and pH is around 47 ppm, and considered too high according to the charts. But it's not, it's fine. Fish are fine and plants are growing lusher than ever. Use the chart as a guide but watch how fish and plants react, then adjust if necessary.