How to Choose the best Auto Top Off for You:

There are a lot of options out there for auto top off’s ranging from simple DIY float switch mechanisms like the DIY Aquahub kit to auto top off systems like the Tunze Osmolator 3155 with its optical eye and float switch backup.  Each system is unique, and a different ATO system may be better for some systems compared to others.  Hopefully, this post will help you identify what features you need for a tank like yours and what the possible options for your type of tank are!



Redundancy is likely the thing you hear the most about with auto top offs and in my opinion it’s the single most important thing to look for.  Adding top off water is critical to your tank but not adding too much is often equally as critical.  If your auto top off adds to much water, it can not only be disastrous for the water chemistry of your tank by reducing the salinity (how much depends on how much water is overdosed), but it also may lead to a flood, potentially causing thousands of dollars in damage to your home.  ATO systems range in redundancy features from nothing to dual sensor combinations and timers.

Dual Sensors:

The most basic form of redundancy is a dual float switch setup like one of the configurations of the JBJ Auto Top Off System or the Aquahub DIY kit. These systems have two float switches installed on them.  You mount one float switch above the other.  When both float switches indicate a low water level, the pump is activated.  When the bottom switch is raised, the pump turns off.  The second float switch is only there in case of emergency, hopefully never being activated. 

Diversified Sensors:

The step up from dual float switch setups is to use two sensors but of different types.  Here the Tunze Osmolator reigns supreme.  By using an optical sensor for its main level sensing and a float switch as its emergency backup, you end up with a higher standard of safety.  This is because the two different sensors sense the water level in various ways.  Something that may cause a float switch to fail isn’t likely to also cause an optical sensor to fail and vice versa.  If a snail crawls on a single float switch, another snail may just as well crawl onto a second one.


It’s becoming more and more common on auto top offs as it’s relatively easy to implement and if your ATO doesn’t have one built in, you could add your own.   Timers regulate the amount of time an ATO system can run for.  Some of them are integrated to the pump run time such as the JBJ ATO system.  When the pump turns on a timer starts and in the event the float switch isn’t activated before the timer kicks in, the ATO system will disable the pump and go into fail safe mode, assuming the tank is leaking or sensor is broken. 

Other ATO timers count the number of cycles.  For example, the Elos Digital Osmocontroller activates the top off pump when a low water level is sensed for 30 continuous minutes.  After 30 minutes it activates the pump for a fixed time and repeats.  If this cycle is repeated multiple times without the sensor sensing water, the top off will go into its failsafe mode and disable.

Quick Tip: If you have an ATO system without a timer, it might be worth adding one if it won’t interrupt the normal operation of the ATO.  For example, if you had a Tunze Osmolator you could easily hook the Osmolator up to a solid light timer.  Set the timer to turn on for say 15 minutes per hour (you can adjust this to be just a little more time than your ATO usually needs to run).  That way even in the event of a failure, the most the ATO could run is 15 minutes in an hour.  If you have the timer programmed to only turn on for a minute or two longer then the ATO generally needs even in the event of a failure it would hopefully take days before it actually flooded.  It generally only makes sense to add this if it won’t upset the standard features of your ATO.  For example this wouldn’t be advisable on the Elos Digital Osmocontroller because you would reset its cycle count and the sensor needs to be on and reading low water for 30 minutes or it won’t activate (plus it has its own built in timer as it is). 


Sensor Types:

Traditionally ATO systems were almost always based on mechanical float switches.  These have served well over the years, though they do have some known downsides.  Primarily that it is quiet easy for a snail or other critter to crawl on them and trigger the ATO system, etc.  Just the last year alone has seen a huge influx in sensor options.  Here is a summary of the more common types.

Float Switch:

Mechanical in Operation.  Straightforward and inexpensive, the float switch has been the mainstay in auto top offs from the beginning.  They are effective but are prone to mechanical failure or interference from tank critters, waves, etc. 

Optical Eyes

Primarily a feature of the Tunze Osmolator 3155, the optical eye is an electronic sensor that detects water level.  It has no moving parts making it much less prone to failure or false reading.

Conductivity Sensors:

Present on ATO units like the Innovative Marine HydroFill.  They utilize conductivity probes to detect the presence of water between the probes. These are another method that does not contain moving parts.

Temperature Based Probes:

These are systems like the Elos Digital Osmocontroller.  The probe includes a small heating element and a temperature sensor.  When the probe is submerged the heat dissipates quickly, but when the water level drops below the level of the probe, the probe tip will heat up.  This is sensed by the temperature sensor and this is what activates the ATO cycle.  The benefit to this type of ATO is its extremely small size. 



You also need to determine what you need your ATO to do.  I know this sounds a bit silly at first.  Your thinking “duh, I want it to top off my tank”.  True, but do you want to use your auto top off to dose kalk?  Do you want it to add very small amounts of top off at a time to reduce changes in water chemistry?  Do you have a pump that has a minimum run time, so you prefer less frequent but larger doses of water? The operation of the ATO will make a difference to which type is suited to your needs, for example an ATO that recognizes and adjusts fairly low changes in water level (like the Tunze Osmolator for example) will be better suited then a system that adds a gallon of ATO at a time.

Single Sensor:

Setups like these might actually have more than one sensor (like a dual float switch setup) but the operation of the top off pump is controlled by the position of a single switch.   When the controlling float switch is down, the pump is on, and vice versa.  These types of ATOs are inexpensive and simple.  They do have the benefit of adding a small amount of water at a time but are also prone to being overly sensitive.  As water ripples in the sump the float will go up and down on the waves, possible causing your pump to turn on and off very rapidly.  This is not very desirable and pretty hard on most pumps.

Timed ATO:

Some ATO systems like the Elos Digital Osmocontroller don’t turn off based on sensor input.  The sensor triggers the ATO to activate and then they run the pump for a set period of time.  This generally adds a little extra water, extending the time between cycles so the system isn’t running constantly.  Some systems like the Tunze Osmolator 3155 utilize a sensor that will turn the pump on when a low water level is detected and off when the water level is restored, but also have a minimum run time that will add at least a minimum amount of water to prevent short cycling.

Latched Mode:

An ATO unit that has a latched mode has two sensors, a high and a low.  When the water level is detected to be below the low sensor, it will activate the pump.  Instead of turning off when the low sensor detects water though they will continue to pump until the water hits the high sensor. 

These can come in a variety of forms such as a simple dual float switch ATO like some configurations of the JBJ Auto Top Off, or systems like the Innovative Marine HydroFill which utilizes conductivity sensors.  Some of these types of ATOs like the two mentioned have entirely separate sensors, you can basically adjust the run time and amount added by changing how far apart the probes are.  The downside is that they can’t get too close together, there has to be a discernable difference between them for the unit to work correctly, though if you’re looking for something that adds a very small amount of water, a unit that works in latched mode likely isn’t what you’re looking for.  Usually you’re using one of these because you’re looking for something that adds more water at a time. Also keep in mind that most units that operate in latched mode don’t have a backup sensor, they have two probes both of which need to be running correctly for the system to work.