It's a relative comparison. If you said you had 600GPH turnover from your sump in your 120g then you'd have a decent turnover for your sump but that would be paltry in comparison to your actual flow needs. If you said you had 6,000 GPH in your 12,000G tank, well sure 6,000GPH might sound like a lot until you actually figure out how much current you'd need in that 12,000g tank. So unless you have a 6,000GPH return pump on your 120g, but then the water would be ripping through your sump so fast that the important stuff probably wouldn't be happening. Like if you add a chiller to your tank and the manufacturer recommended a 400gph pump. Well adding a 800gph pump does not make the chiller twice as effective, in fact it makes it half as effective as with the right pump. There just isn't enough contact time to transfer the heat to the coils and then blow it off with the fan. So would you even bother calculating the 400GPH into your systems configuration? If you want 3,000GPH internal tank flow then I'd say no, why bother?