I found out your bettas tailbiting problem so stop with melafix because it is not him biting. Most likely it is the tetras, they are nippy and aggressive in small schools. Get a 10 gallon tank fast and do a bit more research when it comes to stocking the tank. Neons, even with fishkeepers keeping nano tanks, they usually won't put neons in tanks under 10 gallons, 5 gallons is harsh for them. A 2.5 is small with such an active fish. When you get the 10 gallon, seed the filter if your 2.5 has a filter and bump their school up to at least 6 no less, and that is pushing them to still be nippy. You should do 10 neons and one betta, another school of fish would mean a 15 gallon.
The higher amount of fish for the tank is also going to make your ammonia and nitrites to be higher, especially if you have a small filter that usually come with the tank.