^+1funkngroovy said:They are little amphipods or isopods..
They are eating the algae off your glass. They live in all tanks and are an important part of the biological filtraltion system that we try to create in marine tanks.
When I drained my last tank to move it, the last bit of water was thick with these little critters. BTW, they will also become food for the corals as they will be captured by the feeding corals.
Definately nothing to worry about.