ok good. If you are still doing daily water changes, it might be a good idea to do them less frequently. Get a water testing kit to keep an eye on the ammonia and nitrate levels. Also, keep in mind that it is possible that the fin rot will get worse before it gets better, just keep an eye on it. Also, since you said his smaller tank had no decorations at all, it is very possible that he was tail biting as well. They will do that out of boredom, and it may have become a habit. If that was the case, it will take a long while before it gets better, keeping him entertained with lots of decorations (they can be outside the tank) and visiting him a lot, will help break the habit of tail biting.