I recently bought a 125 acrylic aquarium with a sump off facebook marketplace. The overflow has 4 holes. I figured 2 of them are returns and one is input to the tank from the sump. There is a 4th hole which is puzzling me. Looks like another return? But it has only a notch in the pvc. I don't know its purpose. Does anyone have experience with this? This is my first sump system.
Thank you in advance!
Thank you in advance!