Years ago I had a similar situation occur (only in a much smaller tank, with
only three fish affected). I had only done a bit of a clean up in the tank
and changed the height that I had a powerhead set at. I noticed the fish
getting a bit excited when I moved the powerhead as a bit of scum had
accumulated behind it and it drifted free. Two Rainbows and a Purple SG were
dead the following morning without a mark on them. I still don't know what
got them but I can only guess that it was something they ate. No other fish
were affected.
Just another possible angle to investigate.
Matthew
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