What External Pump would I need?

[Xtremeclownfish]

Hi ALL

I'm in the market for an external pump for my return.
Building a clownfish breeding system, that being said the system will house clownfish, no corals or inverts.

The main tank water volume is approximately 51.96 gallons
Sump approximately 20 gallons
Max head pressure will be 6 ft.

Question? What external pump would I need in terms of GPH to run this system? Appreciate recommendation. FYI I'm not looking to put a hole in my pocket, economical.
Thank you