im looking online on craigslistfor 90+ gallon tanks and i keep seeing like 120+ gallons and it saying things like 30 gal sump. whats a sump? im not new to thefish hobby but i am new to big tank because i onl have a 10 gal tank but going to uprade to a 90-120 in like 2 weeks.

A sump tank is a large tank that has another tank (the sump), typically smaller, that is connected to it in some way, usually by PVC pipe. This sump tank not only increases water volume, but also allows you to put all of your filter media, heaters, and protein skimmer (if saltwater) somewhere out of view. Sumps are typically for saltwater tanks.