Fishsticks, a good rule that I heard, that seems to have treated me well is:
- 10-20x tank size / hour
Reef - 30-50 or 60x tank size / hour
- 30 gallons x 10-20 = 300- 600gph of flow
Reef - 30 gallons x 50-60 = 1500 - 1800 gph
I personally have a 54 gallon corner reef. I started with (2) Koralia 750's, then added a third, and then a forth brining my total to 3000 gph
+ my return pump. Using the 4th one, simply due to the placement in the tank, it made it difficult to get good flow - it actually reduced my current flow in the tank.
For a fish only tank, you should be fine, though I will say that getting nice "Random" flow in the tank is much easier with multiple powerheads. The more flow the better IMO