IME none at all except to collect additional nitrates. I used to have sponges in both sumps and once again they tend to be places that nitrates just love to collect and grow. Bio balls can be tricky. I have heard a few people recommend them for FOWLR systems but as I don't have one of those I am not really sure of the benefit of them. Regardless I believe that the best biological filtration is going to come from having lots of live rock - period. I do have miracle mud in my fuge on my 230g but to be honest I am not sure of the benefit of it (other than that claimed by the manufacturer). My other fuge is just live rock and chaeto.
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