Siva hit the nail on the head there. I never noticed before and I should have asked. I thought you only had a Betta in the tank. That is a lot of fish in a five gallon especially as its still cycling. Chances are that it's never really going to be able to keep up with that bio load even after it has completed cycling and you will continue to have water quality issues with it. How is the 90 gallon going? I would suggest you move the Neons into it. I'd bet if you did then this little five gallon would be fine in a few days, still a little small for a Betta but that's debatable I guess.