Probably fin rot or ich. I would put him in an isolation tank and do like a 25-50% water change on your tank
If it is Ich then separating the fish is pointless, the whole tank and every fish in it needs to be treated.
How big is the tank?
How long has it been set up?
Is it cycled?
What type of filter has it got?
Has it got a heater?
Whats the water temp?
How often do you change water?
How much do you change?
What water conditioner do you use?
What other fish are in the tank?
What do you feed them?
How much do you feed them and how often?
I do it the natural way by raising the temperature slightly
Slightly means nothing, sorry. When giving advice about adding medication or adjusting heat please be specific
When using heat to treat Ich you need to raise the water temp to 30 deg c and keep it there for 4 days, you should see results in 12 to 18 hours if you dont then some salt can be added
Members.
When giving advice about adding medication or adjusting heat please be specific, Right now I am dealing with a member who has a 2.5 gallon tank and she has been adding too much salt to it.
She was instructed to add 2.5 tsp of salt to the tank but what the person helping her forgot to mention is that you dont add 2.5 tsp at every water change.