Mark, they just create nitrates like wet drys, bio wheels and under gravels. Then water changes must be greater to remove the nitrates. However your correct in that they are very efficient at doing their job.
I would not use any of them unless needed for biological filtration, {less live rock and no sandbed.
Something like a Jaubert plenum would provide all the bio filtration needed, plus no nitrates. If so wished, a dsb and shallow sand bed, {somewhat}, will provide the same.
If I was doing a fish only, I would use an abundance of live rock to provide most of the bio filtration.
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Doug
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