Different fish species react differently to high CO2 levels. I found that the smaller, active fish would head for a corner, usually a corner where the CO2 concentration would be the lowest, and look like they were gulping air from the surface. Some less active fish would just lay on the substrate, losing their colors. When my fish clustered in a corner I found that I was very close to losing some - I did lose some at times. So, that was what I looked for the most. Now I know that I should have worked harder to keep the oxygen level in the water higher, which allows the fish to live with a higher CO2 concentration.