As the rev pointed out, they can collect detritus and cause higher nitrates. This is not a problem as long as you rinse the bioballs in SW
on a regular basis. Bioballs do a great job of supporting the bacteria that convert nitrite to nitrate. However, they do not do a good job at removing nitrate. Many people remove the bioballs and replace it LR
rubble. This is a much better media for nitrate removal.