pump to small?

[mytank90]

i just finished pluming my first ever drilled tank <was surprised no leaks> my question is i have to turn ball valve about 3/4 closed for my return pump to work and keep up with flow threw sump .is this normal?i bought a used pump off a canreef member and was told its a mag12 but it has no markings on it .its being a real pain to get system to work without overflowing or running dry depending on where i have valve.tank is drilled herbie style with a 1and 1/2 hole for drain and 2 1 inch return holes