The Salty Side: Saltwater Aquariums for Beginners

Auto Top Off Unit for Saltwater Aquariums

You may have heard of an ATO system or unit and ‘topping off’ a saltwater aquarium.  But what on earth does it do and how do you top off a saltwater aquarium?  Keep reading or use the quick links below to jump to the relevant section about auto top off units.  

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What is an auto top off unit?

An auto top up unit is a system that will automatically fill up your saltwater aquarium when water is lost through evaporation.  It is also known as an auto top up unit some countries.  

This is important because marine ecosystems require stability.  Coral, invertebrates and marine fish can be very sensitive to changes in their environment. 

Evaporation leads to a rise in salinity as when water evaporates it leaves the salt behind.  This is the reason a saltwater aquarium needs to be topped up with RO water (never saltwater!).

Auto top off systems are not essential.  You can run a tank without one but it will make your life easier.  With an auto top off unit there is no more lugging buckets of water to top off your tank every day.  It’s easy to forget to each day when we have so many other things keeping us busy.  

It will also give peace of mind that your tank will be topped off when you’re away on vacation.  That being said, it is always a good idea to have someone stop by and check on the tank in case anything has gone wrong. 

What is an auto top up unit made up of?


The controller is the brains of the unit.  It receives signals from the water level sensors or float switch.  When the float switch drops, or the water level sensor activates, the controller activates the pump that your have placed in your fresh water reservoir.

Once the fresh water has brought the tank water back to the correct level, the controller stops the pump.

Auto top off float switch

The float switch will move down as level of the saltwater decreases. This will then activate the pump to start pumping fresh water from the reservoir into the main aquarium.


As mentioned above, the pump engages when the sensor detects that your water level has dropped, and will move freshwater from your reservoir into your main tank through a pipe.

Freshwater Reservoir

This can be a bucket with a lid, a large plastic water container or a small glass tank.  I used a jerry can and cut the opening out with a sharp blade.  

Whatever container you use, it should have a breather hole as this would cause a vacuum and cause the whole unit to stop working.  It should also be food grade as you do not want plastics that will leach toxins into the water.

You can also purchase purpose built reservoirs like this one from Amazon.

Please note:  This reservoir must be filled with clean, fresh RODI water.  NEVER TAP OR SALTWATER.

Check out the latest price for Auto Top Off Units on Amazon

What to look for when buying an auto top off unit

Reliability would be the most important factor to consider when choosing an auto top off unit.  The last thing you want is for one to fail and empty a whole reservoir of freshwater into your tank.  Not only could this create flooding, if the water overflow but a dramatic reduction in the salinity of the water.  This can be detrimental to sensitive fish and invertebrates. 

Auto Shut Off Feature

An auto shut off feature is necessary to stop water continuously running into the tank.  With the Tunze Osmolator I run in my tank, it has a 10 minute shut off timer which stops the pump running after this amount of time has passed.

Dual Sensors

It is really important to have a backup in case one of the sensors fails.  A good top off unit will come with two sensors, usually one is manual and the other automatic.  

A manual sensor comes in the form of a float switch.  However as it has moving parts, it can sometimes get clogged due to salt creep, or perhaps even a snail, resulting in the float getting jammed.

A digital sensor is an optical eye that monitors the water level in either the sump or tank.  If the water goes above the level previously set then it will shut off the pump.  

Digital sensors can also fail over time which is why it is recommended that an ATO system has two.  

How to set up an auto top off unit

It is really easy to set up an auto top off unit.  Follow the step-by-step guide below and you’ll be up and running in no time!  Don’t forget that although units may differ slightly, they essentially all do the same thing.

Step 1

Place your freshwater reservoir in your desired location.  This step won’t be necessary if your reservoir comes as part of your tank.  

Step 2

Mount the controller using the sticky pads provided and plug into the power source.  You can also use velcro strips to mount the controller.

set up auto top off unit

Step 3

Place the pump inside the reservoir and connect to the controller.  

How to set up an auto top off unit

Step 4

Now connect the water outlet pipe to the pump and feed it over the top of the aquarium.  This can be secured to the tank with a clamp which is usually provided with your auto top off unit.  

Step 5

Mount float switch in the correct position, according to the instructions that came with your auto top off unit.  This can either be in the return compartment of the sump, if applicable, or in the main tank itself.  

tunze osmolator sensor

Step 6

Fill the reservoir up with RO water.

Step 7

And that’s it!  It’ll be one of the best purchases you make.  No more topping off your tank every day and having the peace of mind that your water parameters won’t be swinging from one day to the next.  

Do however read the following on reservoir and outlet pipe placement:   

Take note!

  1. The reservoir should always be lower than the water outlet pipe.  This is to prevent water continuing to flow into the tank, even after the pump has turned off.

  2. The outlet pipe should never be submerged in the tank water itself.  It should be placed above the water to prevent back siphoning. 

What is the best auto top off unit?

For tanks with a sump

There are a quite a few auto top off units on offer and knowing which one to get isn’t always easy.  My choice for the best auto top off unit however is!  

As you may know, I have a frag tank and a 5ft display tank, both of which use the Tunze Osmolator.  Although a tad more expensive than other auto top off units on the market, it is in my opinion, and many others, one of the best.  

I would only recommend products I personally use or have researched thoroughly.   You can read my review on the Tunze Osmolator here and why I have chosen it for my own two tanks.  

For tanks without a sump

Just because you don’t have a sump it doesn’t mean you can’t use an auto top off unit.   Although I have never tried them myself due to the fact that I have always had a sump, two that come highly recommended are the AutoAqua Smart ATO Micro and the Tunze Osmolator Nano.  Neither have dual sensors.  

The AutoAqua Smart Micro has no moving parts which means that will not get jammed due to salt creep or inverts getting in the way.  It also has an alarm that will sound and light up if the pump runs more than three times the original top off time.

The Tunze Osmolator Nano has a float switch with moving parts but this is covered with a protective guard.  It features an auto shut off where the pump is turned off after 5 minutes of topping off if the original level has not yet been reached.

In my opinion, I would favour the Tunze Osmolator as it features the auto shut off whereas the AutoAqua Smart Micro requires you to be there in order to turn the pump off in case of failure. 

So there you have it!

In conclusion, an auto top off unit replaces water lost through evaporation in your saltwater aquarium.  

Two important features of an ATO is that it consists of a shut off timer and has dual sensors.

Topping off is always done using RO water and never saltwater.

Hopefully you have found this useful and now know what to look for when purchasing an auto top off unit and have a basic idea of how to set one up. 

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