I have seen on the forum that some tanks at one or both ends, have a small compartment the same height of the aquarium with small vertical spaces at the top. I guess that is it for the filter as i do not see any canister piping. So how does this filter works? As there are only openings at the top, i guess that it pulls water only from the top. What about the bottom? Does it only need to be used with a sump/wet/dry filter?