I recently added CO2 injection into my aquarium. Since then, I have lost my male A. cacutuiodes and the female will probably die today. I noticed the female would be at higher levels in the tank since the CO2 injection and I took this as a sign of suffocation. Even then, the CO2 was always in the green range of the drop checker during these times and all the other fish, including an A. agassizi, were just fine. Nonetheless I stopped the CO2, and no difference was noted. The male suddenly died a few days and the female is likely taking her final breaths right now. While these two were probably on the second half of their lifespan, I can't help but feel CO2 might be involved in some way. Could this be the case if the CO2 levels are in the blue-green range of the drop checker and all other fish, including another Apisto, are fine?