help with water pumps

[mark]

Max head is where the pumps rolls off to zero output.

Depends on pump curve, how return is routed but guessing your bulkhead/hose could handle your pump volume from that head. Seems common advice to restrict pump output with a value, to prevent overflows maxing out and tank overflowing.

Often numbers like 3/4" bulkheads rated 300gpm, 1" - 600gph. Could check those numbers against how overflow or tanks manufacturers rate volume.