As a general rule, you want to turn over the entire water volume about 6-10 times per hour. Following this rule, for a 30 gallon tank you would want 300 GPH. Keep in mind that the flow will be reduced by elbows and vertical head height. If a pump says that it provides 300 GPH with a max head of 6 feet, and you have 2 feet of head, you can plan on the flow being reduced considerably, say to about 225 gallons.
I say in general, because the flow will also depend on the dimensions of the tank and the inhabitants. SPS will require more flow while LPS and softies may require less. I would stick with the 6-10 times flow rate and get powerheads or Vortec pumps to supply the remaining flow.