I put all of the fish into a 55gal blue drum for potable water. I then set up my ehiem canister filter (their biggest, essentially a 5 gal bucket with a 2212(?) pump on top) that had been running on the main tank, and it moved some water.
not enough, apparently.
The next morning, I had the floaters. The two scats that have survived were floating on their sides as well.
On a hunch, I lifted the return out of the water and thus created more surface agiation. I'd have sworn there was ample sorface motion before, but the two remaining scats perked up quickly and the moray, climbing perch, and fire eel never showed any problem.
So they died a slow death of asphyxiation.
The spotted scat now shows no problem, but the surviving african/tiger/madagasgar scat has a sore on each side at mid length near the top. It looks like they're going to heal, but he ain't gonna be pretty no more. That must be the end-arterial watershed area.
I makes me wonder about the fish I lost in a 5 gal bucket 7yrs ago, also with a well cultured canister filter. Dead the next morn. I always blamed the new store-bought bucket. I don't think so now. I still mourn that big fire eel.