Most canister filters work off of gravity... in a sense. As you pump water up to the tank, it comes back through the intake in to the canister from the tank. If you hooked it in to a sump, you would get, at best, really low flow. Most likely, it would burn the pump up from being overworked. I'm not sure of any canisters that actually pump in, but I know the XP3 doesn't.
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