Don't put the betta in the main tank again. It's badly infected, it sounds like, and there's no point in just reintroducing those germs into your tank after going to all the trouble to get rid of them.
carbon can either be ignored or changed once or twice per month. It doesn't do a whole lot, you see, and does most of whatever it is going to do within the first 10 days or so. After that it gets covered with slime and rendered useless.
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