Aquarium float switches maintain tank water level by automatically adding water when the level drops below a certain point. These switches don't work alone; the switch is one of several components that make up an automatic top-off kit, which includes an external controller and separate tanks for storing aquarium-ready water to refill your fish tank when necessary.
Aquarium float switches include sensors that detect changes in water level. When you place the switch at your desired water level, a float on the switch keeps a circuit inside the switch from connecting when the water level is high enough. As the water level lowers, so does the float. When it goes below the optimum level range, the float uses a magnet to close the circuit inside the switch. When the circuit is closed, it activates the pumping system to add water to the tank. Most switches turn off when the float rises high enough so that the magnet doesn't keep the circuit closed anymore.
The float switch is only part of the automatic top-off process. It's connected to a controller box that sits outside the tank. The controller box reads input from the float switch to determine what the tank needs, whether it's more water or time to shut off the water. Many of these include an additional safety feature, allowing you to set the maximum amount of time the control box allows the pump to activate; if the float switch fails to communicate that the tank is full, the controller box keeps the tank from overflowing by automatically shutting off after your desired number of minutes.
Water and Pump
For the float switch to be effective, you must have a way of getting water into the tank. This requires separate water storage containers, often called dosing tanks, which hold aquarium-ready water that is free of chlorine or other dangerous chemicals. A pump connects the dosing tanks to the aquarium, and the controller box activates the pump when the float switch communicates low water levels. Without all these components, the float switch system can't refill your tank when the water level sinks.
You're not pigeonholed into one type of float switch; pick which one matches your needs best. Vertical switches hang from the top of the tank and have a float, almost like a fishing bob, that slides up and down as the water levels change. It usually takes up more space than a horizontal switch, which is a bit more streamlined and connects directly to the inside of the tank at the water level line. One side acts as a float, raising or lowering with the water level. Both use the same basic process to keep your tank full.