If they're in an established filter, they need to be removed over time to allow the bacterial colonies in the rock and sand to build up to take over. The problem with bio-balls is, they're TOO good at what they do. They're very efficient at converting
ammonia and
nitrite, but the bacteria that removes
nitrate is anaerobic, and will not live on bio-balls. That, coupled with their propensity for trapping and holding
detritus make them pretty much a very efficient
nitrate factory.
