The skimmer just needs to be water tight. If it can be dissassembled for cleaning, all of the unions, joints, etc. must seal well (just the ones below the water line). Some skimmers have a limitation due to the output plumbing and the height of the sump wall. With a tall sump, the return plumbing may cause excessively dry skimate. However, this can be remedied by adjusting the elevation of the skimmer in relation to the sump.