The Salty Side: Saltwater Aquariums for Beginners

The Tunze Osmolator - Best Auto Top Off for Saltwater Tanks

Today I will be reviewing what, in my opinion, is the best auto top off system for saltwater tanks. The Tunze Osmolator.  Hopefully you will find this guide useful and know that I only ever review products that I personally use.  I would also never recommend anything that is below standard.  

tunze osmolator sensor

There is a reason that the Tunze Osmolator is hands down the best selling water top off (or top up if you’re in the UK) unit on the market.  In fact there are several reasons!

I have run a single Osmolator on three separate saltwater tanks, transferring it from one tank to another as I have upgraded to larger volumes. I can attest to the longevity and reliability over the course of several years.  In fact, my faith in the unit led me to purchase another for my frag tank too.

When keeping a saltwater tank, there is potential for so much to go wrong. With the Tunze Osmolator controlling replacement of evaporated water, you have one less thing to worry about.

What Exactly Does An ATO Do?

The acronym ATO stands for Auto Top Off.
ATO’s are also known as automatic top up units, and they have the single purpose of maintaining your level of salinity.

Salt water tanks are ideally maintained at a salinity of 35ppt, or a specific gravity of 1.025. This salt level in your aquarium water becomes compromised due to natural evaporation that occurs on a daily basis.
When your tank loses water through evaporation, it leaves behind a more concentrated level of salt, because salt does not evaporate but instead is left behind. Consequently your level of salinity increases on a daily basis.

To counteract the rate of evaporation there are two options:

  • Manual water top up
    This would require that you make a mark on your tank or sump at the desired water level and physically monitor the level.
    When the water level drops, you add fresh water to replace the evaporated water you have lost.
    The significant downside to this is the creation of a seesawing salinity.

  • Automatic water top off
    Installing an ATO such as the Tunze Osmolator removes the need for manually monitoring and topping up your water. The Osmolator does this for you. All you are required to do is keep the freshwater topped up.
    Salinity is automatically kept at a stabilised level.

The rate of evaporation varies hugely depending on the volume of tank water, the time of year, and the room in which the tank is situated.

To give you an idea, I have a tank with a volume of 550L which loses approximately 30L of water every 5 days!

Replacing this level of evaporated water on a daily basis would not only be time consuming, it would also make the running of my tank far too laborious.

How Reliable Is The Tunze Osmolator?

The Osmolator has proven its reliability through longevity. It stands up above other ATO’s because it simply lasts.

What Does The Safety Float Switch Do?

The Safety Float Switch acts as a failsafe in the very unlikely event the Optical Sensor fails to work as it should. The float rises with the level of the water. Should the water level rise too high, the float activates a switch which in turn sounds an alarm.
Equally should the water level drop dramatically, the alarm will sound too.

How does the float switch & alarm help you?  

  • Protecting your investment:
    You have spent hundreds if not thousands on your saltwater setup. The absolute last thing you would want is for a faulty ATO to top up your freshwater to a point where your salinity level is compromised. This has the potential to kill sensitive corals which thrive on stability.

  • Protecting your property:
    In all likelihood your saltwater tank is situated centrally in your home or business. A failed ATO has the potential to continue filling your tank until the water starts overflowing onto the floor. This can not only prove costly in replacing flooring or carpeting, but the clean up could end up being pretty extensive too.

  • Unexpected blockages:
    In the past I have had my alarm sounding due to blockages in my inlet. As water is slowed going into the sump, the water level in the return compartment drops and the alarm immediately sounds. This is the equivalent of a live notification.

  • Return pump blockage:
    Ideally speaking you should not let your return pump go unserviced for an extended period, and this should not be the way in which you ascertain service requirement. However, should your return pump become blocked or begin to work inefficiently the water level in the return compartment will begin to rise. This will cause the alarm to sound.

  • Freshwater container empty:
    In our busy lives, even the easiest task of refilling the freshwater reservoir can sometimes be overlooked. If the freshwater does run out, this will cause a drop in the return compartment of the sump and the alarm will sound.

What Does The Tunze Osmolator Optical Mini Sensor Do?

The optical sensor monitors the water level precisely.  This ensures that any drop in water level triggers a freshwater top up.

Unlike the safety float that very occasionally needs to be cleaned of salt creep, I have never had to clean the optical sensor.  It’s very design means that it remains clean and clear at all times.

When the water level drops, the controller is immediately notified. This then activates the freshwater pump that replaces the evaporated water precisely.

What Happens If Both Sensors Fail?

Tunze have made the Osmolator with reliability at it’s core.  Should both sensors fail for any reason, the Osmolator has a 10 minute internal timer that would shut off the entire system to prevent over filling and potential flooding.

best auto top off unit saltwater tank

Where Is The Best Place To Mount The Sensors?

In the world of marine keeping you could be maintaining a system with a sump, or one without a sump. The aim is to place the sensor in an area where the water level does fluctuate according to the level of evaporation.

Recommended mounting point in a sumped system

  •  If you have a sumped system your intention is to keep the working of your tank in the sump itself. Within the sump, you will have a couple of compartments, one of which is the return compartment (where you also place the return pump), and another is the main section kept at a constant level between baffles for the use of a skimmer.

    Do not place the sensors in the skimmer section.

    The return compartment is the ideal position for the sensors as it is generally the smallest and most sensitive to water level fluctuations. introducing the sensors here will ensure that water levels are accurately maintained.

Recommended mounting point in a non-sumped system
  •  It may be possible to mount the sensors within the rear filtration compartments. You will need to choose a compartment that mimics the rise and fall of water water level in the tank itself.

    Do not place the sensor in the compartment dedicated to the skimmer, as this is designed to be kept at a constant water level.

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How Does The Tunze Osmolator Controller Work?

Tunze Osmolator Controller

The Tunze Water Level Controller 5017 (which is included with the unit) is a simple display unit with indicator lights.

The controller is mounted within the dry side of your cabinet in a position which allows you to see the display at any time, and also in arms reach should to need to unplug it for any reason – and you will need to.

Ninety nine percent of the time, the controller will display a green light beside the ‘Level’ sign to signify all is well.

Should the water level rise beyond the required level, an alarm will sound and the ‘Too high’ display will glow red.

When the pump is actively refilling the tank, and orange display light will glow beside ‘Pump on’.

‘Too low’ indicates of course that the water level is lower than acceptable and will glow red.

What Else Do I Need To Know About The Controller?

Do to the quality and preciseness of the Tunze Controller, it does pick up on all water level fluctuations.

Below are a couple of examples of when you will need to unplug the controller:

  • When removing water from the tank, either for water changing or selling a coral or a fish on. The sensor will pick up the decrease in water level and the alarm will sound. This will not occur when water testing.

  • When drip acclimating new fish, coral or invertebrates.

The Tunze Osmolator Metering Pump

The water pump is connected to the controller directly, and when activated pumps water into the top of your display tank through a black silicon rubber tubing.

Tunze osmolator pump

These pumps just work and work and work. Mine has run dry of many occasions and when that happens it needs re-priming. To prime the pump, all that you need to do is turn the pump on it’s side in the freshwater while it is pumping.

This will draw water through immediately.

So There You Have It....

I would not hesitate in recommending the Tunze Osmolator, which is available on Amazon.

It’s reliability is without question, and it’s safety features rank it at the very top of ATO design. They are built for longevity, but should it give out after 10 years, you can buy the individual parts!

This is a true investment worth making.

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