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Guide to Saltwater Aquarium Auto Top Offs (ATOs)

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Last updated on November 17th, 2023


Guide to Saltwater Aquarium Auto Top Offs (ATOs)

Importance of an Aquarium Auto Top Off System

In saltwater aquariums, the most important parameter is salinity, or the amount of salt in the water. For a successful saltwater aquarium stability of the environment is critically important. With evaporation playing a factor, it can be very hard to keep salinity constant throughout the day, especially on nano aquariums. While you could add water multiple times per day, if you forget one or miss a day or more because of getting sick or vacation, this stability can be lost and the salt levels thrown out of whack. This is where Auto Top Offs (ATOs) really shine!

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Aquarium Stability with an Auto Top Off

The saltwater auto top off system is a simple solution to keep salinity as constant as possible with minimal effort. The aquarium ATO uses a sensor placed in the tank or sump, a pump placed in a aquarium ato reservoir or bucket of fresh, RO water, and a controller to bring it all together. When the sensor determines that the water level is low, the controller will turn on the pump and fill the tank back to its normal level. All you have to do is fill the aquarium ato reservoir or bucket when the water gets low. The ATO is one of the simplest solutions to a stable, successful reef tank.

Electronic Aquarium ATO Systems

Saltwater aquarium ATO systems have two main types of sensors, float sensors and optical sensors. Float sensors use a small piece of plastic that simply floats on the water and when the water is low, it will open a switch that will turn on the aquarium ATO pump.

Another type of aquarium ATO sensor is the optical sensor, which uses light to determine if the aquarium water is at the right level. If the water is in front of the light, it will refract and the sensor on the other side will not trigger. As the water level goes down, the light will hit the sensor and turn on the aquarium ATO pump until the tank is at the correct level again.

There are possibilities for both of these sensor types to fail either on or off. Float sensors can get stuck and optical sensors can be blocked or misaligned. To mitigate this, some aquarium ATO systems now offer dual sensors to account for when one of the sensors fail. With this added security comes an added price tag, but no other known drawbacks.

Gravity Fed Aquarium ATO Systems - DIY Aquarium ATO

Another popular, entry-level, option is to use a gravity fed auto top off. This is a fully mechanical, or physics based method. There are no electronic components meaning no failures due to power outrages or electronics failures. The only concerns with this method are if there is a leak, or part of the ATO gets clogged, there is no "smart" components to turn them off.

The gravity fed aquarium ATO uses an aquarium ATO reservoir or bucket mounted above the tank or sump of the aquarium. An airline hose is then inserted in to the bottom of the ATO reservoir, with the other side in the sump or tank. Gravity will then force the water within the ATO reservoir through the hose and in to the sump. To prevent all of the water from draining into the aquarium, a float valve is used in the sump, blocking water from coming out of the airline hose once it reaches the optimal level.

This is a cheap solution to satisfy the DIYer in all of us. Downsides to this are obviously the lack of electronic alarms or shut-offs. For example, an alarm if the ATO reservoir is dry or empty, or an auto shut off should the ATO water run suspiciously long. Another caution is that since the ATO reservoir must be above the sump, there are space constraints. If the reservoir should fall, it could break, dump too much fresh water in to the saltwater aquarium, or worse, it could cause a suction, leading to the sump draining on to the floor.

There are also commercial options available for smaller aquariums.


The Gravity Fed ATO provides a strong, low cost solution that could act as a short-term solution. Though this may work for many, I am a partial to the added safety, and less space constraining electronic saltwater aquarium ato systems.

Below is a list of well known brands of aquarium ATO systems. I personally use the JBJ ATO Automatic Top Off System with only float sensors. Though there is concern about the float getting stuck, which has happened a couple times, it automatically turns off the pump if it has been running too long. If you perform regular maintenance, such as taking a minute every couple weeks to clean off the float and shaft, this will not be as likely of a concern.

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All-in-all, there are few "wrong" options when choosing an auto top-off for your aquarium. Simply different levels of how good do you want. Since salinity is a critical parameter, I think it is worth the money ensuring everything is safe and in line. This is especially true with smaller aquariums. If you are under 100g, I strongly suggest a better ATO to ensure the health of your reef.

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About the Author

Reef Stable Founder John Krenzer

John is a Software Engineer with a passion for saltwater aquariums, as well as the founder and president of Reef Stable. He started in the aquarium hobby as a child with a 20 gallon freshwater aquarium. His interest in aquarium life grew and in 2008, John set up his first saltwater aquarium.

Today, John maintains an over 300 gallon reef tank system, consisting of a 120g reef and a 210g reef. These large tanks are contained within the same system, sharing a sump as a means to reduce total maintenance and increase total water volume.

John writes articles for the blog as a means to learn about more reef aquarium topics. These articles act as a reference for the readers as well as himself. John updates these articles frequently to provide additional information or make corrections as new information becomes available.

If you would like to request an article, tank tour article, or to collaborate, let me know via the Contact Me Page!

About Reef Stable

Reef Stable was initially founded in 2019 as a reef tank parameter log to fill a need. Reef Stable quickly grew, becoming a location to solve all of your reef tank problems as well as a place to learn.

Reef Stable now provides a Reef Blog, Reef Aquarium Guides, Coral Care Guides, Identification and Solutions for Pests and Algae, and Reef Dosing Calculators, in addition to the original Reef Parameter Log.

Reef Stable continues to grow, striving to provide a single location for all your reef tank needs!

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