Auto top-off (ATO) replaces evaporated water automatically, keeping salinity stable and saving you the daily chore of topping off the sump. The catch: every ATO will eventually fail. Either the sensor sticks (overfills the sump and floods the floor) or the pump runs dry (drops water level until the return pump cavitates and burns out).
The right ATO has redundant sensors, a hard pump-runtime cutoff, and a float-valve backup.
Optical sensors (Tunze Osmolator, AutoAqua Smart ATO) detect water with infrared - no moving parts to fail. Float switches (cheap mechanical sensors) get gunked up by salt creep and stuck-open or stuck-shut within 6-18 months. Optical sensors fail less often.
A primary "fill" sensor sits at the target water line. A secondary "high water" sensor sits 1/2 inch above. If the primary sticks on (fills past the target), the secondary kills the pump. AutoAqua Smart ATO has dual sensors built in. Tunze Osmolator includes an optical primary + float-switch secondary.
Tunze Osmolator 3155 ($200-280): the reef standard, dual-sensor, programmable runtime cutoff. AutoAqua Smart ATO ($100-180): newer, cleaner, dual-optical sensor, 30-second runtime cutoff. Apex ATK ($300-400): integrates with Apex controller, optical + flow sensor + reservoir level. Hydros Drive ($150-250): integrates with Hydros, similar feature set.
A 100-gallon tank evaporates 1-3 gallons per day. A 7-day reservoir = 14-21 gallons. Smaller reservoirs run dry while you're on vacation. A reservoir with a level sensor sends an alert when it's 3 days from empty.
Mount the ATO pump (typically a Tunze or Aqua Lifter dosing pump) inside the reservoir. Run rigid tubing or hard PVC from reservoir to sump - flexible airline tubing kinks, slips, and is the #1 cause of ATO floods. Use a check valve on the inflow line if the reservoir sits below the sump.
Once a month: lift the primary sensor and confirm the secondary cuts power within 30 seconds. Pour a cup of water into the sump and confirm the system stops dosing. If either test fails, replace sensors immediately.