*Last Updated: June 2024

The 5 Best Automatic Top Off Controllers [Ranked]

Automatic Top Off Controllers

Electronic water level controllers, osmolators, or automatic top off (AOT) systems, allow you to easily and safely deal with water level evaporation issues in your fish tanks and aquariums.

They are engineered to be effective in a variety of different conditions that occur in freshwater and marine aquarium environments.

Many of the automatic top off machines do not have moving parts, which will decrease the chances of running into problems associated with bacterial film, wave action and lime scale deposits.


Our Top Pick: The Smart Automatic Top Off System

This helps ensure regular and safe functioning of the automatic top off controller.  Let's examine some of the popular auto- top-off machines on the market and see how they compare. 

Comparing Them

Water Level ControllerPriceOur RatingWeightDimensions
The Hydor Smart Level Controller & Automatic Top Off Kit$3.71 lbs5.5 x 1 x 2.5 inches
The Tunze USA Automatic Top off ATO Kit$4.11.7 lbs1.5 x 1.2 x 1 inches
The JBJ Automatic Top Off Water Level Controller$3.91 lbs2 x 6 x 6.5 inches
The Smart ATO Micro - Automatic Top Off System$$4.712 oz6.5 x 6.1 x 3 inches
The Autoaqua Smart Automatic Top Off ATO$$3.91.3 lbs10.2 x 2.5 x 6.5 inches

Osmolator ATO Video Reviews

  • primera
  • Tunze
  • JBJ
  • Autoaqua Micro
  • AutoAqua Smart

Our Top Pick



Ease of Use

The AutoAqua automatic top off system is the best rated ATO system that we have come across. It is an easy to install unit, that is ready to go right out of the box, and does not have any moving parts and is engineered to provide effective real-time fail safe protection.

To set it up all you have to do is adjust the optical sensor to the optimal water level of the sump. It ships with a 3x3 inch DC pump (DP-200 model).

Some people complained that the pump wasn't strong enough for their tanks, while others found it perfectly adequate.

For the most part, users were broadly satisfied with its power, but be advised that it may take some fiddling in some cases to get this working perfectly for you and your particular aquarium set up.

How to install the AutoAqua Micro ATO


  • Easy to set up
  • Includes everything you need right out of the box
  • Small size doesn't reduce effective and helps conserve space
  • Very quiet in comparison to other models


  • Some users complained that the anti-siphon valve needed some ingenuity to get working
  • Some users say that the magnets could be a little bit stronger


  • Allows you to automatically refill your aquarium or sump without the hassle of having to heft heavy buckets of water,
  • Dramatically saves time, effort, money and aggravation-all done automatically
  • Effectively maintains the sump aquarium water levels, preventing damage to the pumps

On the subject of reef-keeping, there are lots of fundamental duties you might want to automate. By far the most sought after feature is a method to top-off the fishtank on auto-pilot so that you can restore water which has evaporated from the aquarium.

Tons of beginning aquarium hobbyists might not exactly take into account just how necessary this can be.

As water evaporates, your aquarium salinity increases within the display fish tank simply because salt doesn't evaporate. The water merely gets to be more and more saline the more the fish-tank owner delays adding fresh water. So either stay vigilant or buy an ATO!

How do they work?

  • Auto top off controllers function by automatically refilling your aquarium using floating sensors that assess water level and then activate the pump whenever the water level drops because of evaporation.
  • This helps to ensure a stable salinity as well as specific optimal gravity water levels so that the pump never runs dry.
  • Automatic top off machines typically include level controllers, floats sensors, mounting brackets, LED indicators column, alert signals while pump is an operation

ATO Glossary

  • RO water - Purified Reverse Osmosis water or filtered plain tap water.
  • Freshwater - water which has absolutely no salt added. This can be essentially tap water.
  • RO unit or RO/DI unit - a system for filtering tap water.
  • Top-off - developed to effectivley replace h2o which has disappeared.
  • RO/DI water - Reverse Osmosis de-ionized purified water or filtered tap water which has by far the most impurities removed. This is actually the kind of h2o which is appropriate for your fishtanks- following its mixture with salt.
  • Auto top-off - an automatic kit to maintain your aquarium's top-off requirements.

Although some would rather top-off their own aqua-systems by hand every single day, you can easily grow to be careless and let up in this particular critical responsibility.

Other people include an automatic top-off system into their system, pulling RO/DI h2o originating from a close by or distant reservoir.

What are buyers saying?

Customers report that you do not require a pump for some of the auto top off machines like the Hydor Smart Level Controller. All you do is plug the pump into the controller's main unit and this will effectively control the pump.

One user writes of purchasing the Hydor Smart Level automatic top off controller for his 90 gallon saltwater tank and was highly pleased with its performance. Further, he was happy that it has a small footprint, that it doesn't use a mechanical failure-prone float switch, was highly water resistant and easy to mount, and features a magnetic section cup for the sensor.

Another user, writing about the Hydor ATO unit, lauds their customer support while using two units on his 65 gallon frag system fish tank as well as 180 gallon SPS reef. He was highly impressed by the 24 month guarantee, which he put to use when one of his units malfunctioned and he required a return. Another user writing about the Hydor suggest that you purchase the Tom's Aqua Lifter Pump because it is a perfect match for the Hydor ATO- cheap and dependable, he claims.

While some users complain about the directions that come with the Hydor, there is some video support that has clear guidelines for setting up your smart level controller.

Besides purchasing a quality pump for the AT), many users say that it's essential to understand the best practices and principles of a siphon.

Final thoughts

If you are looking to purchase an automatic top off machine, make sure that you do your research.

It's important that you review comments on the product pages- this is a really smart way of getting the inside scoop on how other users are working with their ATO controllers in their aquariums.

Purchasing a quality automatic top off machine can save you time and aggravation while helping to maintain optimal water levels.

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The Top 5 Team

We are a team of product researchers that specializes in assembling comprehensive buying guides. Our team has a variety of backgrounds, with a mixture of soft and hard sciences represented. Check out our About page to read more about our reviews and editorial process.

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