The marine amphipods that live in most reef aquariums are amazing creatures that look like tiny little shrimp.
In general they are beneficial to a reef tank and reef aquarists do want to provide them with a place to breed in peace (e.g. a refugium). They scavenge and eat detritus in the tank thus help keep it clean.
They also serve as food for the fish and other tank inhabitants.
Click here to read article about amphipods in the on line Reefkeeping magazine.