An In-Depth Review of the Elos Digital Osmocontroller

By Brandon Schlecht

A Few weeks ago we did an article on choosing an auto top.  Over the course of the next few weeks I will be doing a more in-depth spotlight on each of our most common auto top offs to help you identify which has the features you are looking for.  First up the Elos Digital Osmocontroller. 

The Elos Digital Osmocontroller is a one of a kind auto top off system.  With a probe that barely measures 1/3” in diameter, it is by far the smallest form factor sensor on any auto top off system.  We have a few of them setup across the office and its one of our “go-to” models depending on what type of system its going in. 

The Elos Digital Osmocontroller includes a small DC pump to move water from your storage container to your tank/sump.  I prefer the DC pump because the low current DC pumps don’t require you to submerge a pump that is running full line voltage.  Sure most of the equipment in the tank is running at 120v AC right into the tank, but if the DC option is available I prefer it.  The pump measures only 4.25” tall and 1.5” across which makes it ideal for fitting into small locations.  The pump comes hardwired to the main controller unit.  While I haven’t actually experienced any problems with the pumps and its not likely you would have a snail or rock in your top off container, it would still be nice if they were easy to remove if you needed to replace one.

The main controller itself is also quiet small coming in at about 3.25” wide and 1.5” tall.  It has built in mounting slots on the sides of the controller for screws.  The controller has 3 indicator lights, “LEVEL” which remains solid on when water is detected and flashes when low water is detected, “ALARM” which activates when the system enters alarm mode, and “PUMP” which lights up when the pump is active.  A nice feature that I haven’t seen on any other top off systems is the manual fill button.  When you hold the button down it will activate the pump until you let go.  A handy feature for when forget to fill the container and have to catch up on a bit of top off. 

The water sensing of the Elos Osmocontroller is done by a unique temperature sensing mechanism.  The probe has a small heater as well as a temperature sensor.  Being the great conductor of heat that water is, when the probe is submerged the small amount of heat generated by the probe quickly dissipates into the surrounding water.  When the water evaporates and exposes the tip of the probe the heat no longer has the water to quickly dissipate the heat. This causes the tip of the probe to heat up and the temperature sensor in the probe detects this. 

Once the low water state has been detected the green indicator light on the controller will start to flash green.  If this continues for a full 30 minutes the computer will turn on the top off pump (while the pump is active the LED indicator labeled “PUMP” will light up).  The controller will turn the pump on for a fixed amount of time and turn off.  I would like to tell you exactly how much water it will pump each cycle but this will vary on head pressure and your individual setup.    If the amount of top off water was enough to submerge the end of the probe the level indicator LED will go back to constant green.  If the amount of top off water wasn’t sufficient the LED will continue to blink green and the process will repeat itself over the course of another 30 minutes.

In the event this process repeats itself 5 times and the probe has not come back into contact with water the controller will put the system into alarm mode.  The “ALARM” indicator light will turn on and the pump will be disabled.  This is because the system assumes that something has gone wrong.    The most common reason would be that your ATO reservoir is empty, but it could be that your tank is leaking, or you didn’t quiet add enough water on your last water change.  In an event the alarm mode can be reset by power cycling the unit.

This alarm state shouldn’t happen under normal circumstances as you would have to have a really high level of evaporation (like a really large tank) to the point that your evaporating more in 30 minutes then the pump can turn on to compensate for.  If you have a very large tank or one with a really high amount of evaporation, this ATO system isn’t going to be your best bet for this reason. 


Setup of the Digital Osmocontroller is pretty straight forward.  The whole unit only comes as 4 parts, the main unit/pump (there wired together from the factory), the probe which connects via a USB type connection, the power cable, and the bracket.  The unit also comes with tubing for the pump.   Basically the parts plug in where they fit, with the only connections being the probe and the power supply into the controller. 

The first thing you want to do is figure out where you’re going to mount the controller, keeping in mind the cord lengths (58” on the probe cord, 3ft on the pump cord, and a 6ft power cord).  Once you have the controller mounted place the pump in the top off container and run the tubing to the sump or the tank.  Either works, just be sure your pump from a lower location to a higher one, as to not create a siphon.

Next you’re going to want to mount the probe. The probe should be mounted in the return pump chamber of the sump as this will be the area where the evaporation actually lowers the water level.  The Elos comes with a thin plastic bracket that slides over the lip of the sump.  Once the bracket is on the tank the sensor is secured to the bracket via the included zip ties at the height you desire. 

The bracket itself isn’t the fanciest mounting solution of any top off system but it does the trick and its low profile.  I think one of the nice features of this particular auto top off is that you don’t have to use it though.  The probe is small and easy to zip tie to a variety of things so for a lot of the smaller tanks around the obvious it is attached to various probe holders, brackets, etc. 

Once the probe is mounted you will want to plug the other end of the cable into the main controller of the ATO.  The system is basically setup at this point, you just need to plug the power cable in. 


The Elos Digital Osmocontroller is a great option for most.  It is the smallest sensor of any ATO system and its flexibility in mounting the sensor make it great for a variety of tanks.  This is why it’s one of the very few (or only) options  that could be used well with a sumpless tank (most top off systems mount their sensors so far down from the rim that the water level would be an inch or two below the trim).   The fixed volume of dosing can be both a blessing and a curse.  On the plus side it means that the pump is always going to run for a minimum amount of time.  This prevents short cycling of the pump and should extend its life.  By the same note it may make this ATO not suitable for very large tanks as if the evaporation is too great the pump may not keep up and set the ATO into alarm mode.  If you’re interested in dosing kalk in your ATO it has some ups and downs as well.  The nice thing is that you are always going to get a consistent amount of kalk dosed into the tank.  The downside is that if your tank is small, its likely not well suited to dosing kalk via this ATO because it may add too much to your tank each time. 

There might be issues to consider if you use kalk on a very small tank or own an exceptionally large tank, but for the vast majority of folks with tanks big and small, the Elos Digital Osmocontroller is a fantastic, easy to setup, easy to mount, easy to use option.  In typical Elos fashion it looks pretty sharp too! 

Do you own an Elos Osmocontroller? If so share what you think of yours below! If your interested in more articles or (the rest of the ATO series) and want to see more, we do them every week and you find them all right here.