My betta fish has fin rot, but is in a divided tank with another betta?

My male Crowntail,Betta has fin rot. I recently bought a five gallon divided tank, and another betta to go in the tank. This is when he developed the

My male Crowntail,Betta has fin rot. I recently bought a five gallon divided tank, and another betta to go in the tank. This is when he developed the fin rot. I have the MelaFix Fin Tail Rot Infection Treatment, but how do I treat him with another betta in the tank that is just fine?
Backround Info:
Each fish gets 2.5 gallons
Filtered/Heated
Had the Crowntail for Three Years
Have had the newer one for three months

or:My male Crowntail,Betta has fin rot. I recently bought a five gallon divided tank, and another betta to go in the tank. This is when he developed the fin rot. I have the MelaFix Fin Tail Rot Infection Treatment, but how do I treat him with another betta in the tank that is just fine?Backround Info:Each fish gets 2.5 gallonsFiltered/HeatedHad the Crowntail for Three YearsHave had the newer one for three months

or:I would first test the water for PH and Ammonia with a test kit, to make sure the water is ok, if it\u2019s out of sync it can cause problems and stress for the fish. Does the medication have instructions? Otherwise, take the fish out and place in a different tank to treat. Then drain the water from the current tank and clean everything well with hot water. Clean the filter too. The bacterial disease is apparently quite contagous.

Tags:fin,