Salt is not really good for FW fish. It does make their kidneys have to work a whole lot harder. Now this isn't saying it will "hurt" the fish especially short term and there are many that use salt as a preventative in FW tanks. The main thing salt does is irritate the fish's skin which makes it produce more of the slime coat. I do not however recommend the use of salt in a FW tank unless it is specifically needed. Example would be ICH. Warm, very clean water is all that is needed to heal most fin damage, even that cause by fin rot. If the fin rot were to get worse, I would suggest an antibiotic because at that point there is an infection going on. If your tank has a cycled filter, you do not need to remove him water change. If you do not have a filter, then removing him while you do a 100% water change is what you should be doing. You can easily teach your betta to come to the surface and then suck him up in a cup by allowing the surrounding water to carry him into the cup. Pretty soon he will get used to this and not try to get away.
