What are the factors that will cause water mildew and what measures should be taken to avoid it?

What are the factors that will cause water mildew and what measures should be taken to avoid it? If you already have water mildew, how should it be treated? -- mechanical damage to fish, low water temperature, two conditions. There are not many drugs with better effects, methylene blue, or how much is the banned malachite green-formalin-formalin?