You need to read up on how to do a water change. I have an article on my website you can check out if need be under "Cycling a Tank." Basically, in order to get rid of the fin rot, you will need to be changing out about 30-50% of the tank water every couple days for awhile and adding new, pre-treated with water conditioner, water to the tank. Fin rot usually only shows up in tanks where the water conditions aren't good. This means you need to be doing more regular water changes. Posting your water parameters would also be helpful. If you don't treat fin rot, it can eventually be fatal to the fish and all the fish in the tank will get it.