I've kept and bred blue rams. They seem to be one of those fish that is affected by pH.

I've bred them in alkaline water but the eggs never hatched. In acid water (6.5 pH or so) they were easy to breed and the young grew fast. (The alkaline water was also harder, and that could have contributed.)

But more to the point, the population kept in acid water always looked more robust than that kept in alkaline water. It might be that it's not the CO2 that's causing the problem, but the pH that is a function of the that.

Just a personal observation . . .

Bill