Wet/Dry's are waste of time in Marine tanks - IMHO. Add Biowheels to that list as well.
Sumps are bad enough, but wet/dry's also encourage evaporation at a much more drastic rate than a sealed tank, and any experienced reefer knows that adding water to a tank is something you try to do as little as possible given that R/O units aren't free.
Ammonia and Nitrite reducing bacteria will grow in plentifull supply is a layer of Walmart colored gravel with a small amount of current on them, or live rock. Why in $%^& you'd want to move this process to a mechanical unit outside of the ecosystem is beyond me. Plus, live rock and gravel/sand substrate will have *some* nitrate reduction in an anaerobic zone, which you need all you can get in a marine (or fresh) tank. This is sure not going to occur in a Wet/dry unit.