If the hole is at or above the water line you'll get a bit of a spray/squirt thing going on, not a big deal if your lights aren't right on the tank. If the hole is below the water enough water needs to be siphoned back to the tank to allow the hole to be exposed to the air to break the siphon. I kept mine just above the water IE maybe 1/8th inch and it worked well. To adjust your water level in relation to the siphon break you just adjust your overflow height.
Doug
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