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I have them at the bottom of the tank beacuse I have the two filter outputs staggered. One flowing out about 4 inches from the top and the other about 10 inches. Then the powerheads at the bottom. It helps with the fish waste. We have to reposition them to get the poop off the back glass though. But it also depends if you have bottom dwellers or fish that do not like current.
No, since the crab is a bottom dweller, I would not put it at the bottom. Then again, I'm not farmiliar with them and how much current they can tolerate. It would depend on where your filter out put is.