The average recommended turn-over is 5 times, so a minimum 1000 GPH pump would seem appropriate depending on type of fish.
Swamp or oxbow dwellers could probably handle less flow (species like discus, angels, some anabantoids a 500gph might suffice.
Species from fast flowing rivers, counter current zone, more (species like African Tiger fish, rheophillic cichlids, like Tomocichla, Chuco, maskaheros) might prefer triple the flow)