Lost 2 cardinal tetras, one a couple of days ago and one tonight. Could it be the co2? I was told as long as the fish is not at the surface or floating around they are probably ok. But they all do breathe more rapidly (especially my tetras) when co2 has been on for a couple of hours. I have decent surface agitation. Coincidence or co2? None of my other fish or catfish seems affected except for a little more rapid breathing.