A few questions to help diagnose.
How long has the tank been set up?
How often, and how much water do you change during water changes?
Is your tap water hard or soft?
And as asked above what is its pH?
What is pH immediately before a water changes, what is pH immediately after?
Is anything aggressive going on between any of the fish?
Most fish diseases and maladies happen due to some kind of stress (unless a new infected fish was just added), and any of the above questions could answer whether or not one or more of the answers causes that stress.
If you don't know the pH of the tap, it can sometimes be found in the info section of your water bill, or you can simple call your water provider and ask.
How long has the tank been set up?
How often, and how much water do you change during water changes?
Is your tap water hard or soft?
And as asked above what is its pH?
What is pH immediately before a water changes, what is pH immediately after?
Is anything aggressive going on between any of the fish?
Most fish diseases and maladies happen due to some kind of stress (unless a new infected fish was just added), and any of the above questions could answer whether or not one or more of the answers causes that stress.
If you don't know the pH of the tap, it can sometimes be found in the info section of your water bill, or you can simple call your water provider and ask.