Two options are sump and canister, which has already been said.
A hang-on-back pumps water into the hangy box, where it flows through media before going back into the tank.
A sump is the same idea as a hang-on-back. Just, instead of the tube going over and into the tank, it’s usually a hole drilled into the glass (someone can help you), with PVC pipes going down to a tank under the stand (the filter area.) This lets you do cool things like refugiums, skimmers, and a place to put misbehaving hermit crabs.
Obviously you will need to buy the PVC, a tank (1/2 of display volume is pretty good) and an overflow box (this allows your water outflow pipe to suck in surface scum and blocks fish)
A canister is the same idea as the hang on back, just with reeeaallly long intake and outflow tubes, and the hangy box itself can sit under the stand because of the long tubes. The downside is there is much less to upgrade and basically no space to hide a skimmer or anything - in the future you would have to do a hang on skimmer or something. However it is very simple with no drilling required. Secure the intake and outflow onto the glass. Prime the filter - get water into the tubes and body using whatever method that filter uses. Plug in, and you’re off to the races.
