please help me understand plumbing

[Reef-Geek]

Originally Posted by Cal_stir

the hole is the break the siphon if the pump stops and reduces the amount of drain back to the sump, the hole is usually just below the water level, sounds like you are getting a venturi effect drawing air into the pipe

no wonder, it should be below the water level



what should I do to fix it? I can raise the water level in overflow by adjust the drain pipe's height, but I cannot raise it too much...