How Does an Aquarium Float Switch Work?

Float switches are often used alongside sump filters in marine aquariums.
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Many saltwater aquariums feature DIY sump filters. While useful, these filters can encourage evaporation and can cause a flood if the plumbing gets clogged, leading to too much salt in your water or a wet, nasty mess in your home. There's a simple, easy-to-install option to help over come both these issues: an aquarium float switch.

Float Switches

Several float switch choices exist, but all work on the same principle. A floating piece will open or close contacts based on how high or low the water level is. This turns a circuit on or off. In most cases, these switches are used to turn aquarium pump circuits on or off. Float switches help automate some of your aquarium chores, saving you time and trouble. Additionally, a float switch can control certain emergency systems in marine aquariums that can prevent flooding.


Just like topping off your coffee, a float switch can help run an automated top-off system for your tank. Evaporation causes problems in marine aquariums, especially when you use a sump pump. When water evaporates, it leaves salts behind, increasing the salinity of the existing water. Saltwater fish need water chemistry to remain stable, and increasing the salinity too much can stress or kill fish. Topping off the tank with purified fresh water can fix this. A top-off system consists of a float switch wired to a water pump that's connected to a reservoir of purified fresh water. When enough evaporation happens, the switch closes, and the pump turns on. This pumps water into the sump until the float switch opens, breaking the circuit and deactivating the pump. This allows the aquarium to top itself off, avoiding changes in salinity so you don't have to obsess over your aquarium 24/7.

Overflow Prevention

If something goes wrong with a sump filter's plumbing, you can wind up a flood, an electrical fire or both. Sump filters consist of a second water container that effectively increases aquarium volume and gives you a place to install equipment out of sight. Typically, they return water to the main aquarium via a pump and are fed by a gravity drain. If the gravity drain is blocked, the pump will continue to feed water into the main aquarium, overflowing it until the sump is drained, at which point the filter can overheat and burn out. A float switch can turn off the return pump in the event that the main aquarium begins to overflow, preventing disaster.

What to Buy

Finding float switches online is convenient, with the equipment delivered directly to your door. Not all pet stores sell float switches, but you can find them at some electronics supply stores and high-end aquarium equipment. You have two choices of float switches: vertical and horizontal. Vertical float switches consist of a tube with a floating ring, while horizontal float switches have a single bar-like piece with a hinge that opens or closes based on the water level. In both types, you mount them so the switch is open when the aquarium is too low when using them in a top-off system. For emergency shut-offs, you mount the switch so that it is open when the tank is too full.

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