The water level should be at a point where when the pump shuts off, it can hold what continues to drain without overflowing. Put siphon break air holes or a check valve in the supply line if it is lower in the tank than the overflow to prevent back siphon through the supply.
My balls are completely out of water when running, but do get partially covered when the pump gets cut off.
My balls are completely out of water when running, but do get partially covered when the pump gets cut off.

