most sumps are bleow the tank in a cabinet. They need to have an overflow box to bring water down, and a return pump to send the water back up. The water that is shot p gets directed with fow nozzles to optimize flow. In a sump it goes through filter pads, protiem skimmer, and even reactors if you wish. THere are often buble traps to removee micro bubbles. Most sumps even have refugiums with LR, LS, and chaeto. Many people even have Auto top off devices, a swith thet detecs low water and triggers a pump to turn on and pump in water from a seperate water containing reservoir. Sumps have loads of benifits. They are superior to other methods of filtration, the airate the water, for water changes, you dont take water from the tank you take it out of the sump so the livestock are unharmed, and you can probably make one yurself using glass or acrylic. For a 30 gallon sumps are not needed, but i would have one nonetheless. With a 30 gallon you probably only need a 10 gallon sump which would probably take out the use of reactors or skimmers, but you could definitly have a place for filtration media like pads or pellets (phosguard, purigen, matrix...) and even a mini built in refugium. Best way to go for pristine tank water.