[pogodzib]

Ideally, the anti-siphon hole should be above the bottom of your overflow weir, yet below the water level when the pumps are on. When your pumps are running, your tank's water level goes up until your pumps output equals your overflow weir capacity at that depth. The anti-siphon holes should be under the pumps on water elevation so there is no air being pushed into the tank. When your pumps are off, the tank's water level will go down until they get to the lowest point on the overflow weir so you want your holes to be at or above that point. I hope that makes some sense.