My external skimmer has overflowed a few times over the 3 years since my tank has been set up. Even if I keep the cup drain open and let it go into a 5 gal pail it it will occasionally surprise me with a full pail and a wet floor in my basement sump room. I ordered this fine looking solution to my problem. http://www.avastmarine.com/collectio...oducts/djsl-xl