What is an ATO?

ATO stands for Automatic Top Off and is an extremely useful device that automatically replenishes your saltwater aquarium with fresh water as needed.  This eliminates the need to constantly fill your saltwater aquarium manually with fresh RO/DI water.  You also experience a more stable salinity level and constant water level. 

Why Does my Saltwater Tank Need Fresh Water?

Evaporation is normal for any aquarium, in a saltwater aquarium specifically, we must counteract that evaporation in order to maintain stable salinity and hold a constant water level.  During evaporation, only gaseous H2O molecules leave the aquarium; all of the dissolved salts and minerals in the saltwater stay behind.  If we did not add fresh water to replace that which is lost to evaporation, it would cause the concentration of those salts and minerals to rise in the aquarium.  We would also experience falling water levels which can negatively affect pumps and filters.  

It is important to only utilize fresh filtered RO/DI water measuring 0 TDS when topping off your aquarium. This will ensure contaminants do not enter your aquarium through the source water and eliminate any risk of altering water chemistry via top-off.  DO NOT USE SALT WATER TO TOP OFF YOUR AQUARIUM.

ATO Diagrams

How Much Will my Tank Evaporate Each Day?

Rates of evaporation can vary from tank to tank and this is based on water movement, temperature, exposed surface area (size of the tank), and the relative humidity in the surrounding air. The rate of evaporation increases with more water movement, higher temperatures, more exposed surface area, and lower relative humidity.  

Most aquarium owners will fill a 5-gallon jug once per week on aquariums up to about 50-100 gallons in size, give or take. Larger tanks will evaporate more because of the larger surface area and more water movement and 10-gallon ATO reservoirs are not uncommon for tanks up to 200 gallons.  Smaller tanks will go through much less water per week and then you have the variances that occur because of environmental conditions from tank to tank. 

How Does an ATO Work?

There are two basic types of ATO systems, the classic gravity fed ATO and the more modern electronic ATO systems. Gravity-fed systems are super simple but have fallen out of favor for the electronic ATO systems because they are a bit more reliable and easier to manage.  

Electronic ATO System

Pump, Sensor, and Controller of XP Aqua Duetto
Gravity Fed ATO System
Gravity ATO

Electronic ATO systems consist of some kind of water sensor, a small pump, and a controller. The best ATOs have multiple sensors working to monitor the water level and protect from failures.

The sensor is mounted at the desired water line inside your sump or display aquarium. The sensor is connected to the ATO controller which is also connected to the pump that is submerged in a freshwater reservoir. When the water level drops inside the sump/aquarium, the sensor tells the controller to turn on the pump. The pump moves water from the separate freshwater reservoir into the aquarium. When the water level returns to normal height, the controller shuts off the pump.  

Gravity systems do not require a pump and simply rely on a mechanical float valve to open and close as the water level rises/falls. In this instance, the freshwater reservoir must be mounted above the water line in order for water to naturally flow downhill into the sump or aquarium. While the simplicity of a gravity style ATO makes it attractive, there is no protection against failure. What if the float valve gets clogged or pinned open by a snail? What if the seal gets worn out and starts leaking slowly? 

How Can my ATO System Fail?

This is the biggest risk with an ATO system and does happen quite often which is why the best ATO systems have built-in protection and redundancy. If the ATO fails to fill your aquarium, your water level drops and your salinity level rises. Worse yet, if the ATO system overfills your tank it will lower the salinity and cause an overflow and flood. 

Tunze Osmolator ATO Sensors

Failures can occur for a number of reasons but are most often caused by a failing sensor.  This is why the most reliable ATO systems have multiple sensors, one primary sensor along with a secondary or backup sensor to prevent overflow. Sometimes even a third sensor is used. Sensors can fail because of too much water movement, if they get covered in calcareous build-up or algae, or if crawling critters like snails and hermit crabs make them malfunction. Monitoring the function of your ATO sensors is critical. 

Another common failure is not remembering to fill the ATO reservoir.  This can cause the pump to run dry and eventually burn out.  Some ATO systems have protections against a dry run which notify you if the pump has been running too long or simply shut down the pump after a certain amount of time which works to prevent an extreme overfill and to prevent the pump from running dry for too long. 

You could also experience some kind of malfunction with the controller or any of those "murphy's law" situations; what if a cord becomes unplugged, a tube falls out of place, or becomes clogged? Point is that ATO systems will eventually have a failure and the key to a reliable ATO is what happens after that failure; ideally, the programmed or built-in protections will prevent a serious problem.  

Where Should I Mount the ATO Sensor?

ATO sensors must be placed into the pump chamber of your sump or AIO filtration system. This is the only chamber where the water level fluctuates with evaporation. If your tank does not have a sump and overflow box, you can mount the sensor directly in the display aquarium.

Remember, water movement, crawling critters, and exposure to light spell disaster for most water level sensors so avoid these things at all costs. The Sumpless ATO from XP Aqua is a great system for tanks that don't have a sump because it provides a box to protect the sensors. 

Can I Connect my RO/DI System to my ATO?

This is not recommended because you are essentially connecting an endless supply of water to your aquarium. If anything should fail, that RO/DI system could continually push fresh water into the tank until the problem is corrected. By limiting your ATO reservoir size, you can limit the damage in the event of an ATO failure so most reservoirs are no larger than 10 gallons in total size.

You can make the task of filling your freshwater reservoir easier by running a dedicated line from your RO/DI system into the reservoir.  Control the filling manually with a ball valve and use a mechanical float valve in the reservoir to prevent accidental overfill.  The XP Aqua RO/DI Flood Guardian is also a great device for this scenario because it can help prevent the overfill of your freshwater reservoir using an RO/DI system. 

What is a Back-siphon?

ATO Fill Line Installation with a Siphon Break

ATO Fill Line Installation without Siphon Break

A back-siphon is when the flow of water from your freshwater reservoir into the aquarium creates a siphon and continues to flow after the pump is turned off.  There are a couple of ways to prevent this kind of siphon from occurring. The easiest route is to simply install your ATO so that the end of the fill line in your sump is never below the water line in your reservoir.  This way the water always flows uphill into your sump and a continual siphon will never be created.  

Many ATO systems also rely on a "Siphon Break" which is a little fitting used to ensure a siphon does not get created. The fitting has a tiny hole that will allow air to enter the fill line when the pump is turned off, breaking any siphon.  It is important to install the Siphon Break Fitting correctly inside your ATO reservoir and remember, water will come out of the tiny hole when the ATO is operating so position the fitting in such a way that water does not squirt out of the reservoir. 

ATO Auto Top Off Systems

Do I Need to Turn Off my ATO When I do a Water Change?

Yes, always turn off your ATO system when doing a water change or performing maintenance.  It is also a good idea to mark the desired water line on your sump in order to easily maintain the same water level and avoid moving the ATO sensors or fill line. 

Can I Dose Additives via my ATO System?

It is generally not recommended to put additives in your fresh top-off water.  This is because the rate of top off will vary based on evaporation making it difficult to control how much of an additive is dosed into your tank each day. You most certainly never want to mix multiple additives together in your top-off water. 

In days past, some hobbyists would choose to administer Kalkwasser via the ATO system which does work but you lose a certain level of control over the dosing. This almost always causes fluctuating water parameters because the rate of addition via the ATO varies. Now that we have access to accurate and affordable dosing pumps,  there really is no reason to use the ATO system for administering additives because a dosing pump can do it much more accurately.    

What is the Best ATO system?

The Tunze Osmolator 3155 is our favorite but there are quite a few great options. Check out our Best ATO Systems of 2023 list to learn more!