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Reef aquariums are gorgeous, one-of-a-kind pieces that add tremendous appeal to any room they find themselves in, displaying live corals and other uncommonly seen sea creatures.

In contrast to more traditional fish-only aquariums, reef units rely on a much greater degree of Reef aquarium fitted with auto top-off and float switchesprecision when it comes to temperature, water salinity, pH, and other factors.

For these reasons reef aquariums are extremely popular with hobbyists and other do-it-yourself fans as a way not only to beautify the home, but experience the satisfaction of creating a functional self-contained ecosystem.

Auto top-off and sump systems fitted with float switches are a great way for reef aquarium enthusiasts to maintain the stability and overall health of their aquariums while minimizing time and labor.

What is an auto top-off system, and how does it work?

Auto top-off systems, commonly referred to as ATOs, are designed to keep water levels from getting too low in a reef aquarium. Reef aquariums are particularly sensitive to changes in water temperature and chemistry, so levels need to be kept as constant as possible.

SMD Fluid Controls' FS01 vertical float switch, ideal of DIY reef aquarium top-off systems.

SMD Fluid Controls’ FS01 vertical float switch.

This is especially important as it concerns water pH and salinity. As water evaporates from the main aquarium tank, salt and other chemicals remain and become more concentrated in the water that remains. This can mean big trouble for the creatures calling the aquarium home.

An auto top-off system keeps water levels in check to prevent this from happening, adding more fresh water when needed. A basic system consists of three main components: a float switch, an electronic pump, and a fresh water reservoir.

When water levels in the aquarium are too low, the ATO’s float switch is activated, sending a signal to the electronic pump. The pump then takes water from the reservoir and adds it to the aquarium until levels are equalized, and the float deactivates.

Vertical float switches like the FSO1 and FS12 models are generally preferred, as horizontal float switches are more likely to become jammed (often by wandering snails).

It’s important to use clean, RO/DI (reverse osmosis/de-ionized) water in the reservoir. Chemicals in normal tap water can kill the fragile creatures in a reef aquarium.

What about sumps?

An ATO may be used on its own, or as a part of a larger system called a sump. Broadly speaking, a sump is a water reservoir usually kept under the main display tank. Usually a reef aquarium sump includes mechanisms not just to add more fresh water to the main tank, but also to drain it back into the reservoir when water levels become too high.

In addition to being a reliable stabilizer of water levels, a sump has the added bonus of being able to contain filtration systems, water heaters, and other systems that would otherwise be taking up space in the display. Click here for a good article on reef aquarium sumps.

If you’re a reef aquarium hobbyist and you’re thinking of creating your own ATO or sump system, get your float switch from SMD Fluid Controls. A favorite of reef enthusiasts for years, they have the best switches for a variety of applications and configurations.


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