Fish have long nests and are rotten to death. Why is this?

Parasites cause inflammation in fish. It's basically an incurable disease.

There is only one solution: put a certain proportion of penicillin injection into water (3 60-degree wide-mouth bottles), and at the same time take out the fish and put salt on the affected area to directly diminish inflammation, isolate the fish that are not sick and prevent cross infection.

I have used this method before, and the effect is good, but I can't guarantee that it will all be good. Dead fish is inevitable.