An automatic top-off (ATO) system is a device that automatically top-offs your aquarium with freshwater based on the water level in your tank.

As water evaporates the water level in your aquarium drops and needs to be replenished with freshwater to maintain a consistent salinity level. Aquarium filtration equipment also operates more efficiently and effectively when the water level is constant.

Over the years, manufacturers have devised a few different ways of accomplishing the same task: automatically replenishing your aquarium with freshwater when the water level drops.

Although all ATO systems we will be discussing share the same basic components, they are not all created equal. For an in-depth blog post about how ATO systems work, click here.

You can consider this companion post your official ATO system buyer's guide. I have rounded up our most popular ATO systems to give you the lowdown on what they come with, what they don't and I provide my personal opinion on which units do the best job of keeping your reef safe and salty.

AutoAqua Smart ATO – $149.99

Sensor Type: Optical Sensor

Back-up sensor: None

Pump: Includes a safe, low-voltage 74 gallon per hour (GPH) water pump with 6' of tubing that can deliver water to a maximum height of 6.5'.

Bottom Line: This is a unique ATO system because of its design. The sensor and controller are built into one module that attaches to your aquarium with a magnet. The unit senses water through your aquarium or sump walls and can only be used on clear tanks and sumps. Pricing is on the lower end of the spectrum and everything you need to get up and running is included. It is suitable for aquariums and sumps with wall thickness up to ½".

The optical sensor has no moving parts, which eliminates all of the problems that can arise from mechanical floats. The caveat is you must keep the sensor clean so it can detect the water level through the walls of your tank or sump.

Although it does not have a back-up sensor, it does have a smart internal program (AFS Technology) that cuts power to the pump based on how many times and how long the pump cycles in a given period of time. This feature offers added protection against over-fill and dry-run.

Hydor Smart Level – $89.99

Sensor Type: Temperature sensor

Back-up Sensor: Temperature sensor

Pump: Not included. The Smart Level accepts 110/120 VAC pumps with a maximum capacity of 50 watts and 2 prong plug.

Bottom Line: The Hydor Smart Level is one of the new kids on the block. It uses a unique three-pronged temperature sensor to monitor the water level in your aquarium. This eliminates common ATO problems associated with calcium, algae and water movement. For the money, it is a great unit that includes extra over-fill protection.

The Smart Level has an audible alarm that sounds if the third, "high" water level sensor is triggered and will cut-off power to the pump. It has status indicator LEDs and a magnetic sensor mount which is nice—just be sure to keep the sensor clean.

Hydor's ATO has has a maximum fill time of 10 minutes. You want to be certain the pump you choose can deliver all the water you need within this time frame. Also keep in a mind that the Smart Level has a minimum run time of 25 seconds. This means you must choose your pump wisely, especially if you plan to use this system on a nano reef.

JBJ ATO System – $79.99

Sensor Type: Dual mechanical float sensors

Back-up Sensor: None

Pump: Not included. The JBJ ATO system accepts any 110/120 VAC pump with a maximum capacity of 2 amps.

Bottom Line: JBJ's ATO uses dual float switches to sense the water level in your aquarium. With a low price tag and different operational modes, it is a great value and quite reliable. The controller has indicator lights that tell you the status of the sensors as well as the pump. The floats are replaceable and have suction cup mounts for placement in your aquarium or sump. Anytime you use suction cups in an aquarium, you have to be certain they are secure. If the suction cups become loose or detach, it may cause the ATO system to operate incorrectly.

With a maximum fill time of 14 minutes, slow-flow dosing pumps may not work well on larger tanks. Small submersible powerheads are best. The maximum fill time is adjustable, too, so you can dial it in based on your tank size and prevent even the slightest over-fill and dry-run.

JBJ's water level controller relies solely on float sensors and both sensors must be operating correctly for the system to function properly. Snails and other aquarium inhabitants can sometimes get in the way of the floats. Calcium and algae build-up can also cause float problems, so be sure to keep them clean.

Innovative Marine HydroFill – $198.99

Sensor Type: Dual Conductivity Sensors

Back-up Sensor: None

Pump: Not included. The HydroFill accepts any pump up to 1000W, 8Amp/120V.

Bottom Line: The HydroFill uses reliable graphite sensors to read water level based on conductivity which eliminates problems experienced with mechanical floats. It comes with magnetic sensor mounts for easy installation into your sump or aquarium. The unit has indicator LEDs that give you the status of the sensors and pump with a single glance.

Innovative Marine also offers the HydroFill ATO Pump which is sold separately for $98.99 and is thermally protected so it will not run dry. It moves a maximum of 15 GPH with a maximum head pressure of 10' and comes with a nice mounting bracket to attach the tubing to your aquarium or sump.

With a hefty price tag for both the pump and controller, it may not be ideal for hobbyists on a budget. You can save some money by purchasing a less expensive dosing pump or powerhead. The price of the HydroFill system is on par with other ATOs, but without the added security of a back-up sensor or an included pump.

SpectraPure U.P.L.C. II – $229.99

Sensor Type: Pressure sensor

Back-up sensor: Mechanical float

Pump: The SpectraPure U.P.L.C. II includes a high quality LiterMeter III pump.

Bottom Line: The SpectraPure Ultra-Precise Level Controller II is a pretty awesome ATO system with some really unique features that are not all that widely known but definitely worth the money. The primary sensor activates the top-off based on pressure and it also comes with a mechanical float for over-fill protection. The pump flow rate can be adjusted by 10% increments based on run-time. The pressure sensor allows you to adjust the water level variance up to about 4".

The LiterMeter III pump is probably the biggest benefit of this system. It is really great for situations where your RO container is stored in a remote location. The pump can deliver water up to 60 feet and can draw water up to 25'. It is also self-priming and will not experience any damage if run-dry.

Another really cool feature about this ATO system is that it has a separate "drain-mode" which can be used in other applications to remove water down to a set low level—much like a sump-pump is used to remove water from a basement.

Tunze Osmolator Universal 3155$209.99

Main Sensor Type: Optical sensor

Back-up Sensor: Mechanical float sensor with audible alarm

Pump: Includes a safe, low-voltage submersible metering pump with a maximum head pressure of 7.2' along with 6' of tubing.

Bottom Line: The Tunze Osmolator is one of the most widely used and reliable ATO systems available. It comes with everything you need to install onto your aquarium and has redundant over-fill protections. It is my personal favorite and I have used it on multiple aquariums with great success. For the money, this ATO system is hard to beat.

The electronic optical water level sensor is not sensitive to small waves or water movement so pump cycling is not a problem. The mechanical float switch will cut power to the pump in the event that water rises above the optical sensor. The pump will also turn off if it runs for more than 10 minutes—further reducing the risk of over-fill and dry-run—and has a minimum run time of 20 seconds to prevent pump cycling. It has multiple LED indicator lights that show the status of the sensors and the pump at all times. Since the main sensor is optical, be sure to keep it clean and free of calcium and algae build-up.

The Osmolator comes with two strong magnetic sensor mounts making it easy to install in any aquarium or sump with glass thickness up to ¾". It allows for multiple mounting configurations by using one or both of the magnetic sensor holders and even includes some strong Velcro to attach the controller to the inside of your tank stand.

Ultralife Float Switch – $79.99

Sensor Type: Mechanical Float Sensor

Back-up sensor: none

Pump: Not included. The Ultralife Float Switch accepts 110/120 VAC pumps with a maximum capacity of 10 amps.

Bottom Line: This is your basic float switch and is the best option for reefers on a budget. It utilizes a suction cup attachment for the sensor and comes with a 6' grounded power cord. All of the problems associated with a mechanical float put you at risk for over-fill, so be sure to keep the float clean and secure the suction cup well. The unit will only allow power to your pump when the float drops and cuts power when the float rises with no internal programming or protection of any kind.

The benefits to this unit are simplicity, affordability and long-lasting heavy-duty components. It is also very versatile and can be used in a number of other applications including dry-run protection and over-flow protection.