An automatic top-off system replaces evaporated water from a separate reservoir. In a marine aquarium, evaporation raises salinity because salt remains behind; top off with suitable freshwater, not mixed saltwater. An ATO controls water level, so continue measuring salinity independently.
Choose by sensor arrangement, pump lift, mounting clearance and failure protection. The models below illustrate different designs; older product names and photographs should not be assumed to match a current package.
Four designs to compare
Tunze Osmolator 3155: a legacy model
The pictured 3155 combines a normal level sensor with a backup safety sensor. Tunze’s product page now marks this model no longer available. Check replacement-part compatibility before buying used equipment or remaining stock.
The current Osmolator range includes newer models with different sensors and controls. Compare the exact model’s manual rather than transferring the 3155’s specifications to its successor.
JBJ ATO: check whether it is a controller or kit
JBJ’s traditional ATO uses float sensors and configurable operating modes. The manufacturer’s current kit listing includes a pump, tubing and mounting brackets. An older controller-only listing may not. Confirm the actual contents and sensor arrangement before ordering.
Floats require access for cleaning and protection against obstruction. Having two sensors is useful only when they are installed in the roles specified by the manual.
AutoAqua Smart ATO Micro
The SATO-120P uses a compact optical sensor. Its manual lists the pump, tubing, holder and siphon break, with a maximum mounting thickness of 12.7 mm. Check your pump revision and required lift; tank volume alone does not determine whether water reaches the outlet.
Follow the installation diagram for the siphon break. A pump stopping electrically does not necessarily stop a gravity-fed siphon.
Innovative Marine HydroFill Ti
HydroFill Ti uses conductivity probes to detect water level. Innovative Marine lists the controller, pump, bracket, reservoirs and complete kits separately. Price the complete installation you need and verify the exact pump’s placement and protection requirements.
Installation matters more than a ranking
Place the level sensor where evaporation actually changes the water level—often the return chamber of a baffled sump. Keep it away from turbulence and follow its mounting and cleaning instructions.
Secure the refill outlet and arrange the reservoir, tubing and siphon protection exactly as specified. Keep controllers and power connections dry. Test normal filling, high-level shutdown and empty-reservoir behavior according to the manual before relying on the system.
Choose a reservoir whose contents you can manage if filling fails. A larger supply extends refill intervals but also increases the amount that could enter the aquarium. During a water change, pause the ATO so it does not replace deliberately removed saltwater with freshwater.
Inspect sensors, tubing and reservoir level as part of routine maintenance. No sensor type eliminates the need for supervision, cleaning and an independently checked operating level.
