Melafix isn't the strongest remedy, but is generally pretty safe and if used in conjunction with proper water management, might do it for you. Follow the directions on the bottle for dosages.

However, tail rot is an indication of excess bacteria in the water, so start by removing the filter and if you have gravel in the tank, remove the fish to a bucket of tank water, strip out the tank, then refill it with fresh conditioned water and the tank water in the bucket.

Then, with no filter, do daily water changes of at least 50% until the condition clears up, replacing the Melafix as you go. As long as you water change daily, all they need is an airstone.