Use 0.5% salinity water as pre-caution step (100% water change). Have you been feeding bloodworm? When was you last water change? Any predator around your place? During last water change, did you adjust temp before placing the fish in?

They group at airstone as you said; lying on floor of floating in middle?

4" water depth is ok.

during the time you split the fish. Did you chase the fish a lot with net/hand? It sound the fish is a little stressed up. Space is ok but you can reduce it by using a smaller tub (Quarantine use type) to prove your theory on space, it worth a try. When fish eating happily and sudden change like this means something is wrong along the way. Handling these fish need to be a little more gentle.

Stop feeding for a couple of days and observe any changes. Let me know the outcome please. Thanks

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