Tank help!!!?

Extremely high nitrate levels, but everything else is fine. I don't understand why the nitrate level can be off the charts, but the nitrite levels are low, the ph level is fine, no chlorine, tank looks healthy, no signs of parasites, yet every single one of my new additions to the tank die. It is an established tank, 10 gallon. My molly is doing fine in the tank, and when I use it as a holder for fry til they reach a size they can be in the main tank, they do fine as well. For some reason, any kind of algea eater dies in the tank. I have tried algea suppliments thinking they were not eating enough, but still within about 2 weeks they die. Yet the molly shows no signs of stress. My molly is doing better in the small tank than she was in my main tank. Any ideas why I can't keep any algea eaters goin in the small tank? How can I stabilize the nitrate levels? It is high in both tanks, but my algea eaters in the big tank are doing fine. Both tanks have been established for about a year.