Post all test results (ammonia, nitrite, nitrate, pH, GH, KH, temperature).
Does that puffed up cardinal look to have fungus growth (normal on dead fish), or are the actual scales pineconing?
The other two cardinals look fairly normal, don't really see any external signs. Any external physical signs we can't see or any behavioral symptoms you noticed before death?
Done anything different recently that may have caused this hiccup?
Don't know about everyone else, but in my tanks, even leaving a rotting fish to fully decay, test kits would never detect ammonia or nitrites, not even a hint. I guess the filters have always kept any ammonia/nitrite converted down to nitrates quickly and constantly so that there would be no detectable levels. I would even collect water near the rotting corpses and still nothing. Used 2 different API liquid kits, used a test strip and even had my LFS test (all measuring total ammonia, and my pH is around 7.4). I do overstock though, overfilter and over feed heavily. I guess over feeding most days of the week, the bacteria colonies grew to a larger size to handle those daily nitrogen spikes when I feed, so a little rotting fish is not much in comparison.
