Well what do u want in your sump? u should put ur protein skimmer, heaters/chillers if needed, place to put a carbon or chemical filter, you could even put a refuguim down there. I just started with marine tanks 6 months ago and i built my own sump out of a 20 gallon long tank. I have my protein skimmer 2 heaters and some plants that sre in a little refuguim. Much cheaper to do it urself i only spent about 150$ or so on mine. It only cost that much because my tank wasnt drilled so i bought a overflow box. How big is ur tank? if u want to build a 4 foot sump a 30 long would do great. look up how to build a sump you can get a lot of great ideas on sumps you could build.