A bit of backstory here. So I've had this tank for awhile now, but was given a lot of wrong information. The fish are all old, I was given them by a family friend who was trying to get out of the hobby. One of the tanks he gave me was his 125 gallon. Now, the fish in this tank were originally split between TWO 125 gallon tanks, so it was definitely overcrowded (parents wouldn't allow me more than one of them, but the guy gave us both, so there was nowhere else to put the fish). The guy said it'd be fine, and said I only needed to do large monthly water changes, and the oscar already had HITH when I got the fish. He told me these were injuries and not to worry about it. Well, awhile later after losing a few fish and doing more research, I learned that I was not changing the water nearly enough, and the holes in my oscar's head are not okay. Another thing I've learned is oscars have a pretty high bioload. I assume my redhead cichlids do too, they're quite large fish (though I haven't found a ton of information on them out there). The current tank stock is: -2 Redhead Cichlids (Male and female, female is significantly smaller) -1 Tiger Oscar -1 Jaguar Cichlid (Not fully-grown, but growing rapidly (she was stunted as a baby from what I was told though)) -1 Jack Dempsey Cichlid -2 Common Plecostomus (I believe they're common? They're both about 14" long) I know the tank is still overstocked :,) I'll take the water parameters tomorrow and update this asap (though I know the water quality must've been bad going off of how little I changed the water, and that I only recently realized this wasn't enough). There are 2 marineland emperor 400 biowheel filters and 2 other HOB filters that tbh I don't know what they are, but they're almost as large as the biowheels. I've started doing weekly 50% water changes on the tank instead as well. Anyway, back to the main point of this post; What course of action should I take to treat the HITH? Should I do more/larger water changes? I'm wary of medications because of the plecos, and I don't have another tank large enough to put the oscar in to medicate her by herself. Also, I would say she's probably had this at least a year or so. She acts completely fine, though. Never shown any signs of illness, and the HITH hasn't gotten any worse since I got her, and I've had her for QUITE awhile. So I've definitely made mistakes with this tank, and the fish I've lost have paid the price for it, but now I'm just trying to make any effort I can to improve things for these fish. I've been absolutely charmed by this oscar especially.