As the Captain mentioned, sumps aren't a necessity. Nice... but not required. I run a 46g saltwater tank, with a HOB
skimmer, a Marineland HOT cannister to run carbon/purigen/etc, and a little HOB
Penguin biowheel in case I have to start up a quarantine tank with an instant biological filter. I just didn't want to futz with a sump and figured I'd give it a go without and see how things went.
BUT... as mentioned HOB
overflows are a dime a dozen if you want to go that route.