There are 3 sizes of Ebb & Flo fittings: 1/2 inch, 3/4 inch, and 1 inch. With an Ebb & Flo, you have a fitting attached to the pump that is the fill & drain fitting (The water goes in the fill fitting & back out the fill fitting when the pump shuts off hence the name fill & drain) & you have a overflow fitting (The water fills up the the level which you set the overflow to, then when it goes above that point it drains through the overflow). You need the overflow to be at least twice the size of the fill & drain or the water will fill the tray faster then it can drain out. This will cause 2 things to possibly happen the pump might run dry because all the water is in the tray instead of the reservoir & the water might overflow the tray leaking onto the floor.