Rola-Chem M-2000 Pool Sentry Water Leveler - The Pool Sentry Water Leveler maintains the water level of a pool or spa automatically. The supply water stays on at all times when the Pool Sentry is in the pool. As the float drops, the water turns on. When the float reaches its set height, the water shuts off. The float adjusts to the desired level, usually a point halfway on the tile line or skimmer.