Flood & Drain Fittings Set
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The Flood and Drain fittings
The overflow tube is bigger than the water inlet tube, because the water is only going out through gravity, and water is coming in through pressure from the pump, you could wind up pumping in more water than what is going out the overflow. That would lead to water building up and spilling out the top of your system unless you reduce the pressure (volume) being distributed by the pump.
Once the nutrient level in the tray reaches the height of the outlet screen, the excess nutrient drains back to a reservoir.
The tub outlets supplied with these fittings have a 13mm barb on the smaller fitting and a 19mm barb on the larger fitting.