The 29 or 20 gallon tank is for your sump underneath your tank. You need to measure your stand underneath to make sure you get something that fits. It doesn't have to be pretty, scratched is fine, it just needs to be watertight. The overflow sends the water down to your sump and what ever you have down there (skimmer, filter socks, live rock, bioballs, UV, filter medium, carbon...), then your return pump sends it back up to the tank. I suggest doing a search on MAASt and reading some of the posts. You can get a lot of good info on here.