I think tank size depends on what you plan on keeping. For the normal SPS dominated tank loaded with fish, I would max it out at about 400 gal total water volume. The factors that I come up with this number is by the water volume the skimmer holds (5gal), airpull (40-50scfh), body design (smooth conections, open design, bubble diffuser, box base), amount of water can be fed (3-400gph) and overall performance. Can this skimmer handle a larger system, sure. Would I run it on a larger system than 400gals, no I would get the Reflo 250 and call it a day.