So if I've got this right, everything's been fine, then one day you come in and there's one dead GT and the other GTs are in bad shape, including torn fins? So essentially everything seems fine, then overnight you have this?
Very strong sign one or both oscars don't want the GTs around anymore. Could be they've paired up, are courting, or getting ready to spawn. You say you have two GT pairs? Could be it was the GTs that started it for similar reasons, the GTs got territorial or aggressive, but the Oscars laid into them. Different things could have precipitated it but, again, everything fine, then dead, twirling, and torn fish usually means aggression, not illness.
Don't let the white poo fool you, that can be due to injury and stress. And don't let not seeing the aggression while you're around fool you. It's not uncommon for that to happen at night or when you're not looking, despite things looking innocent when lights are on or you're not in the room.
MrsE88
the GT's possibly were already affected with health issues prior. Furthermore the 2 larger Oscars may have stressed them out more. This is why quarantine is so important. Please seperate the remaining GT from the Oscars.
