I recently picked up a few Yellow Rainbow fish and one had a barely noticeable, what I thought was a light wound, mark on his/her side. Well a day and a half later and the fish was missing the flesh going up from the area near it's vent up it's side to the top fin. Not too sure but there may have a been a red line or two near the area like a showing vein or something, was hard to tell as the flesh was pretty irritated and eaten up looking very much like how a dead fish that's been chewed on by tank mates will look after a few hours but nobody had been attacking it. 24 hours later and I've lost a Dwarf neon rainbow and two other tank mates to the same thing with no obvious behavior signs or warnings otherwise. Do not currently have any pictures but if anyone else ends up affected I'll post something asap. I do a 50% water change once a week, 55 gal tank and no nitrites, nitrate, or ammonia. Moderate alkalinity and somewhat hard water with a PH around 7.2. Any advice or ideas on what this could be would be greatly appreciated! So far nobody else in the tank is showing any signs and all look good. I'm about to feed and see if everyone is still acting alright and/or any signs.