Emergency: Goldfish disease

[CreativePotato]

My uncle has recently finished moving into a house that came with a 400 gallon pond with 9 koi and 6 or so presumed goldfish (no barbels, plus they're smaller, but have a similar body shape). One of the goldfish is looking very lethargic, with torn caudal fin(s), spots on its body that look like red burns, (the fish itself all white save for a red splotch on the head), inflamed gills on one side and a fluffy coating on that same side. It just looks really pitiful and my aunt and I really want to save it (or put it out of its misery, as the case may be). She tested the water and came up with safe parameters except for ammonia and nitrate, which were both a bit high. Help!

[CreativePotato]

Crisis temporarily averted. We went to PetCo and purchased a big bottle of PimaFix.

[thekoimaiden]

Well I can tell you your problem right off the bat. Inflamed gills, red spots. That's ammonia poisoning. You should have bought something like Prime to detoxify the ammonia rather than PimaFix; those herbal supplements don't do much. If you're going to treat a disease, hit it hard and fast.

The root of the problem is going to be harder to fix. The pond is grossly overstocked. Koi should have a minimum of 300 gallons per fish. Nine koi should be in a 1800 gallon pond. They are messier than even goldfish. But the goldfish are fine in that pond. However, the koi should be rehomed.

Right now, the best thing you can do is large daily water changes. Start at about 10% and eventually move up to 50%. You'll need a good dechlorinator. Prime or a pond dechlorinator would be best. Adding live plants would be another good move. Floating plants would be best.

[CreativePotato]

Yikes. She figures it's pretty overstocked, but I didn't know by that much. As an update on the fish, she put it out of its misery the evening after I left their house. We bought the PimaFix because of the cottony stuff on its body since after a straight two and a half hours of research she deduced it was some kind of fungus. She's going to continue treatment of the pond to kill off anything that might still be in there.
Hopefully I can convince her to rehome the koi. All the fish were in there before they bought the house, so they basically got stuck with them.