most sumps are used to hide the protein skimmer, heaters, perhaps setup a small refuge (small section of your sump where you can put more live rock, deep sand bed, or grow microalgae to export excess phosphate and nitrate out of your tank so algea will not grow) etc.
on the smaller tanks you can run HOB (hang on the back) protein skimmers so you would not need a sump.
on the larger tanks you want to run the skimmer in the sump, because these skimmer are quite large and sometimes loud.
don't get me wrong, you can use large canister filters on a salt tank with fish only. but most people go salt to eventually go reef, and the live rock is the backbone (literally) of a reef tank.
on the smaller tanks you can run HOB (hang on the back) protein skimmers so you would not need a sump.
on the larger tanks you want to run the skimmer in the sump, because these skimmer are quite large and sometimes loud.
don't get me wrong, you can use large canister filters on a salt tank with fish only. but most people go salt to eventually go reef, and the live rock is the backbone (literally) of a reef tank.



