ATO systems automate the most mundane maintenance task of replenishing the aquarium with freshwater lost to evaporation. This not only means you won't have to manually fill your tank every day but also ensures the salinity and water level remain stable protecting both your tank inhabitants and equipment. 

What makes a good ATO you ask? Cost, quality, sensor size, and ease of setup are good things to consider when shopping for an ATO system but what matters most is the redundancy. All ATO systems will eventually have some kind of failure and good ATO systems integrate redundant layers of protection to protect you and your tank from an ATO mishap. The best ATOs take this even further and will have built-in alarms to notify you directly when things go wrong.

Shop ATO Systems

Best Selling ATO Systems

Tunze Osmolator Universal 3155 and Reef Breeders Prism ATO

When it comes to auto top-off systems, nobody has executed as successfully as Tunze with their Osmolator 3155. The strong magnet, reliable operation, and redundant protections are what make the Osmolator the most popular ATO system on our website. The Reef Breeders Prism ATO is the runner-up in this category and isn't too far behind the Osmolator in terms of the number of units we've sold. The Prism comes in at a fraction of the cost, 35% the cost of an Osmolator, but still provides a decent level of redundancy and is super compact.


Shop Tunze Osmolator

Most Reliable ATO System

Tunze Osmolator Universal 3155

Having redundant layers of protection is what makes an ATO reliable and Tunze does this particularly well. The Osmolator 3155 uses extra strong magnets to secure the sensors and engage dissimilar technologies side by side so if one fails, it's unlikely the next layer of protection will fail for the same reason.


Shop Float Valve

Best ATO on a Budget

Any ATO System With an added Float Valve Failsafe

A simple mechanical float costs less than $20 and could be the single most important addition to your ATO system. The best part is, it works with any ATO system and simply requires a small hole drilled through the wall of your sump.  Mount the float and connect your ATO fill line, if the ATO fails and the water level rises beyond the connected sensors, the mechanical float will rise and stop the flow of water into your aquarium. 


Shop Osmolator and Solenoid Water Valve

Best ATO for Connection to an RO/DI System

Tunze Osmolator Universal 3155 with Solenoid Water Valve

The solenoid water valve can be wired directly to the Osmolator 3155 and control the flow of water through 1/4" RO tubing. This allows you to run a line directly from your RO/DI system to your aquarium instead of using a pump.  Keep in mind this also means you're connecting a virtually endless supply of water to your aquarium so the addition of a mechanical float is critical for this configuration. 


Tunze Switched Socket

Best ATO for Long Distances

Tunze Osmolator Universal 3155 with Switched Socket Outlet

Tunze's switched socket wires directly to the Osmolator 3155 in place of the traditional fill pump.  You can then plug in whatever pump is necessary to move the water over long distances or to fill a large aquarium.  


XP Aqua Duetto 2 ATO System

Best ATO for Nano Aquariums

XP Aqua Duetto2 ATO System

"The world's smallest dual sensor ATO", the Duetto2 is the second generation ATO from Xp Aqua that was built specifically for tight spaces. The tiny sensor measures only 1" square and contains two precision water level sensors to maintain a constant water level with minimal variation which is perfect for all-in-one style aquariums that can't afford large fluctuations.  Setup is quick and easy using the magnetic coupling and there is no programming required, simply mount the sensor, install the pump and plug it in! 


Auqa Gadget Hydrofill Reservoir - Innovative Marine

Best ATO Reservoir for Tight Spaces

Innovative Marine Auqa Gadget Hydrofill Reservoir

Just when you thought an ATO reservoir couldn't be clever, Innovative Marine delivers the Hydrofill. This glass ATO reservoir looks great and has an ingenious fill drawer which makes it easy to refill without having to remove the entire reservoir from underneath your tank. Simply pull the drawer, pour in the water, and you're all set.  No more fussing with heavy, awkward containers, and say goodbye to spilling water every time you refill your ATO.