how to choose an aquarium ato

How to Choose an Aquarium Auto Top Off

The introduction of aquarium auto top-offs or ATOs has improved tank health by keeping salinity levels more constant and reducing the amount of daily maintenance. However, if not properly maintained, the sensors on an ATO can fail and cause a significant change in the tank’s salinity.

Why Topping Off the Water Level is Important

Topping off the water in an aquarium keeps the salinity stable. Evaporation occurs in all aquariums, and when saltwater evaporates, it leaves the salt behind. Failing to replace this evaporated water regularly will cause the salinity in the tank to increase. Topping off the water level also prevents the return pump and other equipment from running dry by maintaining a consistent water level in the aquarium sump.

To learn more about the importance of salinity within a reef tank, check out This Article from ATI North America.

How to Top Off the Water Level – Manual vs. Automatic

When it comes to topping off your tank, you can manually add RO/DI water daily or invest in an automatic top-off unit. Making this decision is as much about your lifestyle as your tank. If you’re someone who prefers to be very hands-on and is constantly doing maintenance, you may be ok with topping off the tank daily. On the other hand, for those who prefer to automate most of their tank maintenance processes, an ATO is a must-have piece of equipment. Regardless of which option you choose, the main objective of keeping the water level consistent remains the same.

What is an ATO?

An auto top-off system or ATO detects changes in the water level of the aquarium or aquarium sump and automatically replenishes any evaporated water with freshwater. The purpose of an automatic top-off is to ensure that the water level within the aquarium is kept consistent.

How Does an ATO / Automatic Top Off Work?

ATOs use a sensor to detect when the water level inside a sump or aquarium is too low. When the water level falls below the sensor, the auto-top-off triggers a supply pump that pumps freshwater into the system. Many ATOs feature a second sensor that acts as a backup if the first sensor fails.

Aquarium ATO Illustration
Aquarium ATO Illustration

Types of ATOs + Comparison Chart

While there is no shortage of auto top-off systems on the market, there are essentially only two main types; a float valve ATO and an Optical Sensor ATO.

Float Valve ATOs

Float valve top-off systems have been around for a while, and as the name implies, they utilize a float. When the water level falls, the float valve can no longer float, triggering a supply pump. The float valve is pushed back up as the water level increases, turning off the pump. Most systems use a second float as a backup or fail-safe if the first float fails. Float valves are a reliable, cost-effective solution if the unit is regularly cleaned of salt creep and other debris.

Examples: JBJ ATO Water Level Controller and Tunze Osmolator Nano 3152 Auto Top Off

Float Valve ATO Illustration

Optical Sensor ATOs

An optical sensor ATO works by emitting a beam of light and measuring the amount of light returned. When the water level falls below the sensor, the amount of light received by the sensor increases, triggering the supply pump. As the water level increases, the amount of light received by the sensor decreases, turning off the supply pump. Similar to float valve ATOs, optical sensor ATOs generally use a second sensor as a fail-safe if the first sensor fails. Removing salt creep is once again crucial step to ensure proper operation.

Examples: Hydros ATO, IceCap ATO, ReefBreeders Prism+Exo, and XP Aqua Duetto

Optical Sensor ATO Illustration
Optical Sensor ATO Illustration

Combination ATOs

Some of the most beloved ATOs on the market combine the two previously discussed models. The most common iteration of these units uses optical sensors as the primary level setters and a float valve as the final fail-safe to prevent overfilling.

Examples: Neptune Systems ATK V2 and Tunze Osmolator 3155

ATO Type & FactorsFloat ValveOptical SensorCombination ATOsExplanation
Safety Combination ATOs offer an additional level of security.
Maintenance Each method requires the same amount of care.
Space NeededSome Space NeededLeast Space NeededMost Space NeededA float valve typically takes up more space than an optical sensor.
Price While multiple protection methods are the best option, it is also the most expensive.
ATO Comparison Chart

Sizing Options

For most ATOs, the flow rate of the supply pump will determine the recommended tank size for the unit. There are, for example, some ATO models designed for nano aquariums and others that are best suited for larger systems.

Potential Drawbacks of Aquarium Auto Top Off

While an ATO can give you the peace of mind to maintain a consistent water level, there are potential drawbacks. One is that the sensors could fail, leading to a significant change in the tank’s salinity. Secondly, refilling the top-off reservoir with RO-DI must be done regularly. If it’s not, water levels in the tank will drop, increasing salinity. And finally, one advantage of topping off your aquarium’s water level manually is that it can be a great time to get a close look at the health of your system.

Concluding Thoughts on Auto Top Off

An aquarium auto top-off will keep the water level inside the sump and aquarium consistent and help maintain the proper salinity levels. It will also reduce maintenance and is an excellent first step in automating your reef tank.

Please contact our support team if you have any additional questions about ATOs or other aquarium automation equipment.

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