If the cause is not water parameters, the feeders may have brought in disease. All you need is one carrier.
You don't say what size or how old your fish are, but a 125 is much too small for all those, even at mid adult size.
If they are young maybe they can handle it for a little while. But overcrowding will eventually cause stress, and in turn disease.
Sorry to be a downer, but somebody had to say it. You've got large fish, that live in large bodies of water in a puddle.