I would say they are not good bio balls work when water is passed through them. if you want to use bio ball they would be best in a canister filter or in a section where water passes through them to get to the next section. Live rock would be much more beneficial in them in my opinion as it converts ammonia to nitrate and then converts the nitrate to nitrogen which is harmless with its bacteria whereas the bio alls just convert from ammonia to nitrate which is bad at high levels