My Betta fish has gotten fin rot 2 times?

after the first case you should have scrubbed down the tank and treat it with the meds for the fin rot this would kill any fin rot bacteria in the water and ure health if you did do this your tetra may have fin rot but not show any signs because the power of the disease and it's immune system are equally matched so treating the tank with the tetra would hopefully get rid of the disease but you could also try some aqua vitamins to help your tetra. The best thing you can do is 25% water canges every week to insure good water quality and intern this strenghtens your fish's immune system with blocks dieses