Best Auto Top Off System For Reef Aquarium (2024 Full Guide)

Dr. Mollie Newton
Published by Dr. Mollie Newton PHD| Senior Editor
Last updated: June 13, 2024
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We conduct hands-on testing for all the products highlighted in our reviews and guides. Through anonymous product ordering and involving an independent team of testers, we gather direct experience to offer recommendations backed by data.

An Auto Top Off (ATO) system is incredibly beneficial and convenient for saltwater aquarium enthusiasts. It functions by automatically refilling the aquarium’s water level when it gets low. This remarkable device not only saves time but also lessens the physical effort needed from the owner by eliminating the need for manual water top-offs!

However, finding the best automatic top off system can be quite confusing as you’ll need to consider things like your tank size and the features you’d like the unit to include, such as dual sensors or a built-in alarm.

I was equally overwhelmed when I first bought an ATO system for my aquarium, so to make your search a lot easier, I’ve compiled a list of some of the best auto top off systems for an aquarium. I’ve also covered everything you need to know about ATOs further down in this guide if you’re unfamiliar with what they do and how they work!

Article Summary

  • Auto top off systems (ATO) are useful for maintaining water levels in saltwater aquariums, saving time and effort.
  • Consider tank size, auto shut-off timers, redundancy, and features when choosing an ATO system.
  • ATO systems typically include a sensor in the sump connected to a controller, which activates a pump in a freshwater reservoir to replenish water in the tank.

Best Auto Top Off Systems For Reef Aquarium…

Top Pick

XP Aqua Duetto ATO

This smart ATO system by XP Aqua provides two optical sensors to keep your water level consistent, as well as a stress-free installation.

Top Pick: XP Aqua Duetto ATO

One of the best auto top off systems to use for your aquarium is the XP Aqua Duetto Dual-Sensor Complete Aquarium Auto Top Off as it boasts easy installation, an ultra small dual-sensor, and precise water level control in one complete package.

The two sensors work spectacularly to keep your water at a stable level – this, alongside the smart QST programming, also prevents your tank from overfilling. The system also works in darkness!

Everything you need for a simple installation process is included with this auto top off system, including a magnetic coupling so you can both mount and control the primary sensor and secondary optical sensor with ease. No programming is involved, too, which is ideal if you aren’t particularly tech savvy, like myself!

However, this auto top off system can malfunction after around 18 months, so you may need to replace relatively frequently.

That being said, it’s still a great unit that offers a range of useful features, so it’s well worth considering! It also doesn’t have a huge price tag, unlike some other auto top off systems.


  • Weight: 1.04 lbs
  • Power Input: 100-240 V
  • Dimensions: 6.42” x 6.42” x 2.83”


  • Simple installation
  • Dual optical sensor for precision
  • QRT programming prevents overfilling


  • Can malfunction after 18 months of use
Best Value


The JBJ ATO is a decent option for aquarists on a budget – it features two float sensors that activate automatically when the water evaporates in your fish tank.

Best Value: JBJ ATO

If you’re looking for an affordable ATO system basic kit for your fish tank, the JBJ Automatic Top Off Water Level Controller is a solid choice. Despite its budget-friendly price point, it is fairly reliable and works well most of the time.

The device comes with two float sensors that automatically detects when your sump or aquarium water level drops – refills the evaporated water almost instantaneously to your set point! Electronic controller features two modes of operation and has three setups, too.

However, the floating sensor switches can be a little temperamental and get dirty easily, which can impact the performance of the entire device. You may need to clean the floating sensors frequently (a couple of times a week) to keep them in good working order.


  • Weight: 1 lb
  • Power Input: 110 V
  • Dimensions: 2” x 6” x 6.5”


  • Two floating sensors
  • Refills evaporated water within seconds
  • Controller features two modes of operation


  • Float sensors can be unreliable
Editor’s Choice

Tunze 3155 Osmolator ATO

This ATO system from Tunze offers multiple redundancies to make it extremely safe to use.

Editor’s Choice: Tunze 3155 Osmolator ATO

If you can afford to spend a little extra on an auto top off system for your tank, I fully recommend the Tunze 3155.000 Osmolator Auto Top Off System – it’s one of my personal favorite ATO systems due to its fail-safe advanced technology.

It features multiple redundancies for absolute safety and an optical sensor. The latter is backed up by a floating switch, as well as a time limit alarm and pump speed adjustment!

This makes it one of the safest ATO systems you can use for your fish tank as it doesn’t depend on a single technology.

However, as much as I like this auto top off system, it can be a little inconsistent in performance. The float sensor can be a bit finicky, so you may need to adjust it frequently (several times a week) to improve its efficiency.


  • Weight: 2.8 lbs
  • Power Input: 110 V
  • Dimensions: 4” x 1” x 2.5”


  • Fail-safe technology
  • Features several redundancies for safety
  • One of the safest ATO systems you can use


  • Sometimes the sensor fails

Tunze 3152 Nano Osmolator

The Tunze 3152.000 Nano Osmolator is ideal for a saltwater aquarium under 55 gallons. The whole system comes with a single optical sensor, low-voltage pump, magnetic mounts, and an audible alarm as a safety feature.

Tunze 3152 Nano Osmolator

Another great auto top off system to use for your sump or display tank is the Tunze 3152.000 Nano Osmolator, which is suitable for aquariums under 55 gallons. It’s a single compact unit that comes with an audible alarm that turns off the system after your desired time (up to three minutes).

It can be installed either on the edge of your fish tank or in a cabinet filter, so it suits a variety of setups. The ATO sports a 11V power supply, as well as a low-voltage pump, making it energy efficient.

The self leveling controller comes with an 118-inch hose, magnet holders, and even a protective wrap to keep the primary sensor secure.

Unfortunately, the pump included with this ATO system can be a bit loud during operation. It can also be a bit temperamental, so you need to adjust it every now and again to get it working properly.


  • Weight: 1.57 lbs
  • Power Input: 12V
  • Dimensions: 1.5” x 1.25” x 1”


  • Features an audible alarm
  • 12V power supply makes it energy efficient
  • Pump included, alongside a hose, magnet holders, and a protective cover


  • Provided pump can be unreliable and noisy

XP Aqua Ultimate ATO

The XP Aqua Ultimate ATO is ideal for monitoring the water level of large reef aquariums as it comes with not two sensors, but four!

XP Aqua Ultimate ATO

Next up is the XP Aqua Ultimate ATO System, which features four sensors for maximum accuracy – it’s actually the world’s first four-sensor auto top of system. It’s a great choice for a larger tank for this reason.

One sensor is used to detect the water level of your reef tank, another is used for run-dry protection, and the last two are used as an overfill dual alarm system with QST programming.

If that wasn’t enough, the ATO comes with a wide range of accessories to suit a variety of saltwater tanks, including a DC pump, socket switch, and solenoid valve. The latter makes it suitable for RO/DI systems.

Another great feature of this ATO system is its quick and easy installation – no programming is involved, simply plug it in and let it get to work in your reef tank!

The main drawback of this auto top off system is that it can fail after around nine months, particularly one of the sensors. That being said, XP Aqua offers excellent customer service, and are very quick to respond to warranty claims and issues with their products.


  • Weight: 1.6 lbs
  • Power Input: 100-240 V
  • Dimensions: 10” x 8” x 2.02”


  • Quick and easy to install
  • Features four optical sensors
  • Comes with a wide range of accessories


  • Can fail within a year of use

MagTool Optical Sensor

The MagTool Optical Sensor ATO is an affordable but high-quality unit that boasts dual sensors and smart technology. It comes with a pump, mounting implements, alarm system, check valve, and hose holder.

MagTool Optical Sensor

The last auto top off system I recommend for a saltwater or reef tank is the MagTool Optical Sensor ATO, which comes with a good selection of features, especially for its affordable price tag.

It boasts a compact design that can be used for sump tanks, as well as rimmed and rimless saltwater aquariums. It has dual optical sensors that allow it to monitor your water level with the utmost precision.

I really appreciate the smart level technology of this device – it can detect your tank size and the amount of time it will take to fill your fish tank (without overfilling). The included controller will also display visual alert signals and halt power to the pump if it detects that the refilling duration is six times longer than estimated.

The unit can distinguish if the sensor/controller is not working as intended, and if the water reservoir is empty. It even has LED indicators so you know if the system is working properly.

In addition, you get everything you need to monitor the water level of your tank with this auto top off system, including magnetic holders, a hose holder, check valve, and a water pump (70 GPH).

And best of all, you get a one-year free warranty with this auto top off system that covers the ATO controller, pump, and power adapter.

However, the pump included with this auto top off system is very small, so it may not be suitable for a larger tank. Nano aquariums work best with this ATO system.


  • Weight: 13.4 oz
  • Power Input: 100-240V
  • Dimensions: 5.71” x 5.39” x 3.46”


  • Pump included
  • Dual sensor unit
  • Smart technology
  • Comes with magnetic holders, LED light indicators, an alarm, and more


  • The pump included may not be ideal for a large size aquarium

What Is an ATO or Auto Top Off System In an Aquarium?

Auto top off systems are devices that automatically add fresh water back to your saltwater setup. They are extremely convenient as they remove the need of manually topping up your fish tank with fresh RO (Reverse Osmosis) or DI (Deionized) water.

What Does an ATO Pump Do?

An auto top off pump transports water from the reservoir of the ATO unit to your saltwater or reef tank.

How Do Auto Top Off Systems Work?

Automatic top off systems work by using a sensor that detects when the water level in your aquarium has dropped. The sensor then notifies the controller to activate the pump to fill the missing water in your aquarium with freshwater.

Why Do You Need An Auto Top Off System?

An automatic top off system is useful for marine aquariums as they remove the need of manually replenishing evaporated water in your aquarium with RO/DI water. One of the most crucial aspects of running a healthy saltwater aquarium is maintaining your salinity levels.

When water evaporates in your aquarium, the salinity levels in your tank water increase because the elements that do not evaporate become more concentrated, resulting in a rise in salinity. That’s why you need to add fresh water back to your aquarium to maintain the correct salt-to-water ratio.

What Can I Use for an Auto Top Off Reservoir?

You can use practically any water-tight container as an ATO reservoir, including a smaller tank (like a 10-gallon fish tank), water vat, bucket, or a storage bin.

What Is the Best Auto Top Off System?

The best automatic top off system depends on your size aquarium and what sort of features you’re after.


However, I think the XP Aqua Duetto Dual-Sensor Complete Aquarium Auto Top Off ATO is a good choice for small and large aquariums!

What Should You Consider When Buying an ATO System?

Automatic top off systems are fairly complicated devices that can be a hefty investment, so it’s important that you do your research to ensure you select the right unit for your setup. Below are some things you should consider when buying an ATO system!

What Tank Size Is It Suitable For?

It’s a good idea to check what size tank the auto top off system you’re looking at is suitable for, as smaller units will not be powerful enough to monitor the water level of large aquariums.


For example, the Tunze 3152.000 Nano Osmolator I mentioned earlier in this guide is only ideal for aquariums under 55 gallons.

Does It Have an Auto Shut Off Timer?

Automatic top off systems that have a safety shut off timer are useful as they only allow the unit to run for a set duration, which gives you a bit of extra safety if the sensor or float switch on the ATO malfunctions.

Does It Include Redundancy or Fail Safes?

Redundancy involves incorporating backup components or measures to ensure reliability and peace of mind. An auto top-off system with redundancy, such as float switches, dual sensors, or a timer, prevents disasters like overfilling your aquarium.

Adding too much water to your saltwater setup can be just as harmful as adding too little as it will alter the salinity of your water and may lead to flooding.

What Features Does It Have?

Depending on the model, the type of features an auto top off system can vary, but the ones I think are the most important in an ATO system are:

  • Single or Dual Alarm
  • Pump
  • Dual Sensor
  • Reservoir

ATO systems that have a secondary sensor are particularly beneficial as they provide more accuracy than units that only provide one.

Do I Need a Sump for an ATO?

While you can run an ATO without a sump, it’s much easier if you have one as this is where the sensor on the unit should be placed. If you’re set on using an ATO without a sump, you will need to put the sensor directly in your aquarium.

How to Set Up An Auto Top Off System

For most ATO systems, the sensor should be placed inside your sump and connected to a controller that is also connected to a pump. This pump is placed inside a freshwater reservoir. Once the water level in your sump or tank has lowered, the sensor on the ATO orders the controller to activate the pump.

The pump then transports water from the freshwater reservoir to your fish tank until the desired water level is met, at which point the controller automatically turns off the pump.

  • Easy to install
  • Has two sensors
  • QRT programming stops overfilling

  • Time limit alarm
  • Pump speed adjustment
  • Features multiple redundancies

Editor’s Choice

  • Controller has two modes of operation
  • Replenishes evaporated water within seconds
  • Two float switches for quick security technology

Best Value

Final Thoughts

I hope I helped you select the right auto top off system for your tank, as well as helped you understand a little more about how these devices work.

ATO systems are an invaluable piece of equipment for saltwater tanks as they monitor the water level in your setup and add fresh water back when needed.

Which automatic top off system did you decide on for your tank? Be sure to let me know on our social media platforms and share this post with your friends and family!

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