Everlastics Fill/Drain & Overflow Fittings are mostly used for ebb & flow and drip systems. In an ebb & flow system, you pump the water from your reservoir up to your flood table. When the water level gets to the level of the overflow fitting, it spills back into the reservoir from the overflow fitting. In a drip system, you would just use a Fill/Drain fitting to drain the water back to your reservoir from the flood table. The fittings accept 1/2″, 3/4″, or 1″ tubing. By far, the best fittings in the industry!