"Giancarlo Podio" wrote:

It's possible that with the addition of CO2, the plants are consuming
nutrients much faster than before and therefore a deficiency has been
created in one or more elements.

You know, that's quite possible. However, I only started using CO2 about a
month or two ago, and I've started having problems with plants nearly two
years ago. Still, it's an interesting hypothesis.

Have you checked your nutrient levels?

I'm afraid don't know how to do that. I might not be a bad idea to learn
how, though.

What about substrate fertilization? It's possible that your
substrate has also been stripped of nutrients due to the increase in
plant uptake.

I don't use substrate fertilization. I only have ordinary gravel. While I
know it's not ideal, my plants were doing great for the first year. I don't
know why they started doing poorly after that.

What does your fertilization routine look like?

My aquarium is 40 gallons. I add 20 mL of Plant Gro (Hagen's iron-based
fertilizer) about once a week, right after the light turns off.

I appreciate the advice. Thanks.

Francois