I'm afraid the fish's problems are due to the tank - specifically its size, water conditions and being grossly over stocked.
Most of the fish in your tank should not be there. For example, zebra danios should be kept in a tank at least 3 feet long as they are such fast swimmers.
Loaches, tetras and danios are all shoaling fish which need at least 6 of the same species - in a much bigger tank.
If you wish to save the betta there is only one realistic course.
Remove all the other fish either to a bigger tank or rehome them to shop/another fish keeper.
The betta can survive in 2.5 gallons, though bigger is better. Colin_T has told you how to treat the fin rot, then going forwards you need to change at least half the water every week. If your water supply contains chlorine, you can let the new water stand for a day or two and not need water conditioner, but if your water supplier uses chloramine, that does not gas off and you must use a water conditioner to remove the chlorine.
I'm afraid you cannot keep fish in such a small tank without water changes and not have sick fish. 95% of illnesses are caused by poor water quality, and with so many fish in such a small tank with no water changes, the water quality will be extremely poor.