Joux123456 wrote:

What is the best treatmeant for tail fin rot of one goldfish in with 3 other
goldfish and a catfish? Isolation needed? Turn filter off? change feeding?

Don't turn the filter off!

Depends whats causing the fin rot - do you have nipping fish? You're
best bet is to test your water parameters to ensure they are ok
Ammonia, Nitrite and Nitrates. Salt can help with fin rot as can melafix.
Salt solution should be about 0.1% which is *about* 1 teaspoon per gallon
or 1 tablespoon per 3 gallons.