ATO (Auto Top Off)
Automatic evaporated water refill system. Maintains a constant level in the aquarium by adding osmosis water automatically.
Detailed explanation
Evaporation is an inevitable fact in any open aquarium. The water evaporates but the salts remain. If you do not top up regularly, the salinity gradually increases. In marine aquariums even an increase in 1-2 ppt can stress corals.
The ATO resolves the issue automatically. A level sensor in the aquarium or sump detects when the level drops below a threshold and activates a pump that adds osmosis water from a reserve tank.
The most popular ATO systems use optical sensors (Tunze Osmolator, Smart ATO) or float sensors (cheaper but less reliable). Optical sensors have no moving mechanical parts and are more precise.
A good ATO must have at least two levels of safety: the primary sensor and a backup sensor that stops the pump if the level rises too much. A malfunctioning ATO that pumps fresh water without stopping can dilute the salinity to the point of killing everything in the aquarium.
The osmosis water reserve tank must be sized to cover evaporation for at least a week. In a 300-litre aquarium evaporation can be 2-5 litres per day depending on the ambient temperature, surface movement and the presence of a lid.
In freshwater aquariums ATO is less critical because evaporation does not change salinity. However, a constant level is important for the aesthetics and for the correct functioning of the filter (especially if the filter has a weir).
The water added by the ATO must be exclusively osmosis water. Never salt water, never tap water.