An aquarium Automatic Top-Off (ATO) system is a device or set of devices used to automatically add water to an aquarium to compensate for evaporation and/or water loss due to other factors such as splashout, or water being removed during maintenance tasks. The purpose of an ATO system is to maintain the proper water level in the aquarium, which can be crucial for the health of the inhabitants and the stability of the ecosystem.

A typical ATO system consist of a water reservoir, a float switch, a pump, and tubing. The float switch is installed in the aquarium and is connected to the water reservoir. When the water level in the aquarium drops, the float switch activates the pump, which draws water from the reservoir and adds it to the aquarium. When the water level in the aquarium reaches the desired level, the float switch turns off the pump.

Some ATO systems also include sensors and controllers that can monitor the water temperature, pH, and conductivity to ensure the water is within the correct range for the specific inhabitants of the aquarium, and make adjustments accordingly.

ATO systems can be particularly useful for reef tanks, where water levels need to be maintained within a specific range and sudden changes in water level can be dangerous for delicate coral and other inhabitants. They can also be useful for planted tanks and other aquariums where the water level needs to be kept consistent.