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Mostly, what a wet/dry filter is, is a filter, that has bio-balls in it to filer out the water...some people sub. bio-balls with crushed coral.....but if your starting a coral tank and you have enough lr, (at least 1.5lbs/gal) then anysort of bio-media would be a waste because the rock is your filter...the use for a wet/dry filter then is to break up the water before it gets to the sump...
Bio-balls are very nice to have, but if they are left in your filter and not cleaned, it can be very farmful for your tank...after a while bio balls hold on to bacteria for so long that its a very bad thing....
Reef rubble is a better substitution than bio balls or CC IMO. I have had my converted wet/dry setup for about 8 months with this change and I have 0 nitrates. Just make sure if you use the RR in the wet/dry, it stays submerged, unlike bio balls.