Hi Lordsameth - From what you're describing, it appears that all the treatment information that @earthworm88 has described is the best treatment for your health issue. In regards to your 5 gal, it sounds as if the fin rot/fin melt is an outcome of poor water quality. Although your tank is cycled, it's important to do regular, weekly water changes. For a small 5 gal, 50% would suffice. Small, varied water changes with one regular change every 2 weeks is unfortuantly not enough. It sounds as if your little guy is experiencing bacterial issues and the number one cultprit is poor water quality. Especially if you are uncertain of the water parameters. Cycled tanks require a weekly water parameter check to insure that the water changes are frequent enough and the percentage of water taken is also sufficient. Our little bettas can produce a pretty big bioload. Even in my cycled 25 gal, I do a weekly 50% water change. The best health we can provide our fish is clean water. I learned this lesson the hard way. I used to do small, varied water changes with my first cycled 10-gal and had no idea my goldfish was suffering. Her behavior was completely normal one day and then the next day she was riddled with bacteria sores and died within 4 days. Bettas are tough little fish so don't let his behavior fool you. I'd QT your little friend, treat for fin rot, do a nice 50% water change in your 5 gal with a vaccum, check your parameters, make sure the ammonia and nitrites are 0ppm and nitrates are under 40ppm and then return him to his home when he's well.