What corals are you intending to stock your tank with? The reason that I ask is because I've never had any problems with my scarlet hermit crabs bothering ANY of my corals (other than walking over them and occasionally trying to steal the food from the coral's mouth).
I guess if there is excess food settlement in your overflows, the crabs may take care of it, but the circulation in there would have to be mighty slow as crabs won't be able to catch the food as it passes by. A fish seems to make better sense in this situation.
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