Best Aquarium Auto Top-Off (ATO) Systems for Reef Tanks (2026)

XP Aqua's Duetto 2 uses a proprietary dual optical sensor to hold the water level, two layers of overfill protection and QST programming meant to prevent overfilling, according to its listing. It is plug-and-play with a magnetic coupling for mounting, uses stainless steel parts, and the listing calls it the smallest dual-sensor ATO system.

Mount the sensor in a sump's return section or at the display's waterline, set the level, and fill the reservoir with RO water.

XP Aqua's own page for the Duetto 2 (model DATO-202P) lists a 74 GPH pump with 8.2 feet (250 cm) of head that draws 4.8 W at 12 V DC, a sensor mount for glass up to half an inch (12.7 mm), and 6 foot cables. XP Aqua says this second version adds controller programming and circuitry for added protection and a more durable sensor, and that the sensor works in complete darkness. It asks you to keep the sensor away from micro bubbles and direct light, and to give every cord a drip loop.

Tunze's Osmolator 3 (model 3154) puts two independent sensors in one housing: an optical sensor that reads the water level to 0.5 mm (0.02 inch), and a thermal safety sensor at the top edge of the housing. Water carries heat away far better than air, so when the water rises far enough to wet the safety sensor, the sensor cools, detects it and cuts power to the pump, overriding the optical sensor; once the level falls back, it releases the pump on its own. Tunze names two limits: cold drafts below 59°F (15°C) can trip it falsely, and water above 95°F (35°C) may not cool it enough to react, so keep the protective cap on.

The last line of defense is a 10-minute safety timer that starts every time the pump does. If neither sensor has stopped the pump after 10 minutes, the timer does, and you unplug the unit and plug it back in to restart it. Tunze asks you to keep enough spare room in the tank or sump to hold 10 minutes of pumping. At the standard setting's 70 liters an hour over 1.2 meters (3.9 feet) of lift, that is 11.7 liters (3.1 gallons), which is a lot of spare room in a small tank, so run that math before fitting it to a nano reef.

The included Turbelle High Jet 5030 dosing pump lifts water 3 meters (9.8 feet) at the standard setting and 6.2 meters (20.3 feet) on boost, which Tunze suggests for refill water kept in a basement, tube runs over 3 meters, or systems over 350 liters (92.5 gallons). It stops if it runs dry and restarts itself after 5 minutes, has blockage protection, and needs a reservoir opening at least 75 mm (2.95 inches) wide with the standard connector. The sensor holds on glass up to 15 mm (0.59 inch); mount it where the water can rise to the top of the housing without overflowing. Tunze gives no upper tank size for the Osmolator 3. Its smaller sibling, the Osmolator 3 Nano (3151), drops the safety sensor and relies on a refill timer alone, which is why this page picks the full Osmolator 3.

FZONE's ATO uses a small optical sensor (0.79 inches across) for rimmed, rimless or sump tanks, a micro pump rated at 74 gallons per hour with a 2 meter lift, a check valve, a 6 foot tube and magnets for walls up to 0.78 inches, according to its listing. Its QST 2.0 protection stops the pump and sounds an alarm if refilling exceeds three times the preset limit, and it detects sensor faults and a low reservoir.

Kamoer's ATO One 2 SE uses one optical sensor to detect when to refill and a second, mounted higher, that only acts to prevent overflow, plus a refill timeout alarm in case a sensor fails, according to its listing. An optional third sensor watches the reservoir level. The listing says the newer version uses an anti-dry pump and more accurate optical sensors, and warns that the reservoir should not sit more than 1.5 meters from the tank in height.

Jebao's Jebato-150 detects the water level with a compact optical sensor on a magnetic mount, refills from a reservoir with a low-voltage DC pump, and includes a siphon-break part to stop backflow, according to its listing. It lacks a second overflow sensor, so keep the reservoir small: a small reservoir is a failsafe of its own, because it limits how much fresh water can ever reach the tank.