it hang on the side of the tank like one of the hangover filters but instead as the water level rises it over flows into the box thru tube's into a sump thats under your tank for filtration. i am not new to fish but new to sumps and have recently found out they are the best way to go its so much easier. and u can make a sump out of to buckets its so cheap n easy. only reason i am selling is because i only bought it to try out overflows n sumps and now i have a drilled tank so i dnt need the overflow attachment only the sump.