When I first started 3 months ago I had problems with ammonia due to the cycling process, I lost five neons in as many days. They all died within 24 hrs of first showing simptoms.
But more information would be needed to make a better diagnosis. Post your test results, details of how often you do water cahnges and how much, and anything else you can think of. even if you think it might not have anything to do with the problem.
Maybe then someone can be of more help.
Happy Fishwatching
DKC