I have a two hundred that has been set up since 1999 I run a filter pad on top of my bio balls and nitrates are below 5. As a matter of fact I had to add nitrates a couple of weeks ago because I had no nitrates and my corals started to crash. On my 158 set up since 2003 I do not run a filter pad because my corals do not do well if i do. The nitrates are below 5ppm in this tank. I do not know if something got stuck in your bio balls and was creating ammonia? Nitrates can only survive if they are fed in the nitrogen cycle. If you run bio balls then run a filter pad on top, because to the best I my knowledge that is how Dan designed the wet dry system.