[bertoni]

Bio-balls are very effective at converting ammonia to nitrate. That's very handy in a fish-only system. Most reefkeepers want to keep nitrate low, and the bio-balls seem to cause problems along those lines. There are some theories, as to why, if you want to do some reading.

You could try removing the bio-balls slowly, over a period of a few weeks, at least. I'd watch the ammonia level carefully.