07-16-2011, 04:21 PM
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the issue i see with the sponge is that i think it would allow for to much to collect and break down that it might be counter productive. i think you would have to rinse the sponge to frequent to keep any good bacteria alive. i cant knock it till i try it, this only seems logical to me. i think ill stick with sand.
the gravel, crushed coral would work well too, however the cracks in these substrates are to large, and the flow of a refuge is just to slow to prevent it, and they turn into nitrate issues.
for the record, i personally would avoid crushed coral, gravel, bio balls, bio wheels, and anything that has a chance to collect debris and offer time for it to break down into excess nutrients like nitrates and phosphates.