theyangman
Distinguished Member
- Joined
- May 22, 2013
- Location
- London, Ontario
The hole is to allow air to enter the return line and break the potential back siphon in your return line as fast as possible. It prevents your display water ending up in your sump and potentially overflowing it (in extreme cases) Depending on how low the exits of your return line are situated in your tank are this can mean several inches of water.