Are Auto Top Off Systems just for convenience?

Published 2018


Written by Dan

We have all experienced it; someone offers us a drink and when we take that first sip of our Tea, Kool-Aid, etc. we instantly realize it is too sweet, too watered down, too stout, etc.  The amount of water in our drink has either diluted the drink to much or not enough.  Our tank inhabitants experience this same feeling each and every time we top off our tank.

Like so many people in the hobby I went years just filling up my tank when I noticed too much evaporated out.  With some of my systems this was not as big of a deal.  I saw the water level drop in the display and I just put some fresh water in before it got so low my hang on filter wouldn’t work.  After I had been in the hobby a while I started to use water conditioners to dechlorinate and buffer the water before I added it in.

Once I started into Saltwater tanks though, the chemistry of the water became more of an issue.  As the water evaporates the salt doesn’t go anywhere.  This means the salinity of the tank increases with every drop that evaporates out.


If this was true about the salinity and salt than what about all the other elements and particulates in the water.  As the water evaporates the level of contaminants in the water becomes denser.  Man, I never thought about it that way.

What does that mean for my fishy friends.  Well, shifts in the water chemistry are going to stress your fish out.  Fish naturally protect themselves against pests and disease with the layer of mucus that covers their body.

A stressed fish is going to have lower production levels of this mucus and as a result a thinner layer of it covering them. The thinner level is going to be easier for pests like ich and flukes to attach themselves to your fish.  This is no different than how cold weather doesn’t actually make us sick, but it impacts the body making us more susceptible to illness.

It will impact our invertebrates even more drastically.  Many invertebrates have thinner skin and some even allow the water to flow through their skin.  They also are heavily impacted by the osmotic pressure of the water.  Shifts in the pressure level, salinity, and such will impact and, in some cases, directly kill them.

Corals will be impacted the most by water chemistry since lots of them feed on the water column and rely on trace elements in the water to grow.  Many also maintain a layer of mucus just like fish that keep them protected.

So how do we keep the water level consistent to prevent this change in chemistry from happening.  Well easy answer is to add more water more often.  Well we all work, have families, and let’s face it there is no way we can add water to our tanks throughout the day as it evaporates.  At least, not long term consistently.

Enter the Automatic Top Off System, commonly known as the ATO.  The ATO was invented by TUNZE in 1985. 

The ATO uses a switch system that keeps track of the water level in your tank.  If the water level drops below a designated point, it will pump water from a reservoir into the tank until it is back at the designated level.  Once it is back at the designated level it will shut off the pump.


BOOM! Problem solved.  Now instead of having to keep track of the water constantly, I just check my water reservoir and fill it up every few days.  After a few weeks I actually really was able to get a grasp on the evaporation rate of my tank and know how often I needed to check and refill my reservoir.

Auto Top Off system differ widely with several different methods of doing this.  I have seen some that tap straight into the water supply of your house with lines like the ones that feed your ice maker of your fridge.  They are run to the tank and a float ball opens and closes a valve allowing the water to flow as the float raises and lowers with the water.

I am not a huge fan of these because there is a much higher risk if it malfunctions and does not shutoff.  With an infinite amount of water able to flow to the tank feeding straight from your homes plumbing a malfunction could mean hundreds of gallons flooding over the tank, maybe even thousands of gallons if it happens while you are on vacation.

I like the systems that utilize a reservoir tank.  This means there is a limited amount of water that could flood your tank if your float switch malfunctions.  The downside though is that you do have to periodically fill your reservoir.

With this style of system, the float rather than mechanically opening a valve will trigger a pump to turn on.  This pump will move water from the reservoir to your tank.  Once the water level raises back up the float will trigger the switch turning off the pump.

Although float switches have been the most common water level detector for years we have started seeing new technology in the market.  Systems that use LED light detection and even electrical conduciveness like in a TDS meter have become popular as well boasting that they do not suffer from issues like floats sticking in the on or off position like classic designs.

These technologies have their own pros and cons as well though.  LED sensors can fail if covered in algae which blocks the light or the light sensor.  TDS style systems wont work in pure RO water since it carries no current.

Ultimately having some sort of ATO is going to simplify maintaining the water level in your tank and make life easier for you.  It is going to keep your fish healthier because it aids in maintaining a consistent water chemistry.


Personally, I use the Tunze ATO Nano Osmolator on my 180g Reef Tank.

First of yes I know this model is rated for a 55 gallon tank.  The thing you have to remember is that I am not actually topping off the 180 as it evaporates.  I have a 40 gallon sump that holds about 20 gallons of water during normal operation.

This means I am really toping off the water level of a 40gallon tank.  The float valve and pump level operate on the water level of the 40g not the 180g.

If it was on the 180 the sheer amount of water that would evaporate before the float moved would mean the pump could not keep up and is why this tank is rated up to a 55g tank.  Since I run it on the 40g sump it works perfect.

Now we know the water is still evaporating out of the 180.  Therefore, the water level in the 40g drops more frequently and causes my Tunze Nano to kick on more often than if it was topping off a 40g on its own.

That logic may not work in all cases but has worked well in mine and several others I know using this same pump for even larger tanks.

Something to keep in mind with all models is to make sure you keep the outlet of the water hose above the inlet of the pump.  This will prevent a siphon from forming which will drain your reservoir completely.  This is a common mistake that many people encounter not thinking that once the pump kicks in to start the siphon that gravity will keep it going even after the pump turns off, just like with your gravel cleaner.

So, what is the best ATO system?  If you are looking to buy one of these systems this is definitely a question you will be asking.  There are several very popular models on the market that all work well.  Each having their own pros and cons.

My personal favorite is the Tunze ATO and the Tunze ATO Nano.  Use these links to check the current price of them on Amazon.

Let’s take a deeper dive into each of the popular options to see what they have to offer.


Tunze Auto Top Off Osmolator 3155

Tunze is actually known for creating the ATO system in 1985.  They have continued to improve the system since that time.  The standard size ATO is rated for any size tank.  At low head pressure (3ft) it will pump 26.4gph.  At 6ft it will pump about 15.3gph.

The Tunze ATO is ready to go right out of the box.  It includes sensor, controller, and pump as well as the needed hosing.  The only thing you need is a reservoir of some sort to hold your water.  I have found the standard 5-gallon jugs sold for use in the hobby work great for this since the pump actually fits in the opening of the jug.

The Tunze uses an optical mini sensor to accurately identify the water level turning the pump on and off as needed.  For redundant security a float sensor is also present to shut off the pump if the optical sensor fails and the water level raises above the optical sensor.

The sensor mounts easily in any tank or sump using a magnet.  Place the sensor at the desired water level against the inside wall of your tank.  On the outside place the magnet to hold the sensor in that location.

The controller uses a simple 4 light display to indicate:

  • Water level is Low
  • The Pump is on
  • Water level is correct
  • Water level is to high

The controller also has an audible alarm that will sound if the water level is to high notifying you of the issue.

With the ability to handle large tank sizes coupled with the accuracy and added overflow prevention securities this is definitely my favorite model.  You do get what you pay for.

Check the current price of the Tunze Auto Top Off Osmolator 3155 on Amazon.


  • Pump included
  • Capable of handling large tanks
  • Pumps up to 26.4gph
  • Can handle higher head heights allowing you to pump all the way up to the display tank from the floor
  • No additional pump required
  • Tunze is a very well known and trusted company with a great track record in the industry for many years


  • This is one of the priciest options, but offers a lot more than its competitors
  • If the sensor is placed in an area where algae grows or salt creep accumulates the optical sensor will need to be cleaned periodically to ensure it works properly
  • Connecting pump to controller is not done with standard outlet plug

Tunze Auto Top Off Osmolator NANO 3152

The Tunze Nano ATO is the little brother of the standard size.  The good news is the Nano 3152 packs just as big a punch as its brother the 3155 since they both use the exact same pump.

This means that the nano can also pump up to 26.4gph.  The differences in the two models is in the controller and water level detection.

The nano uses a single float switch to determine if the water level is to low.  If the float drops to the bottom of its float range it kicks in the pump to fill back up.  If it reaches the of its float range it shuts off the pump.

To prevent over filling the controller operates on a set timer.  If the pump stays on for either 2 or 4 minutes it will cut off and must be unplugged to reset this.  To select between using the 2 or 4-minute timer you open the controller box and adjust the jumper switch.  Covering both pins with the jumper sets it to 2 minutes and only covering 1 sets it to 4.

That sounds complicated but, once in your hands it is easy to figure out and is generally only set once. Unless you move it to a different sized tank.

The 2-minute setting is intended for tanks up to 26 gallons and the 4 minute setting for those up to 55.  With this unit being able to pump up to 26.4gph; doing the math shows in 2 minutes it would fill just under 1 gallon and around 1.75 gallons in 4 minutes.

Now keep in mind if you let your reservoir run dry when you fill it back up the water level may have dropped lower than the .9g it can fill in 4 minutes.  Simply unplug and plug back in the controller to finish filling.

Connecting the pump can be a little confusing at first glance, but is really easy once understood.  It is done by plugging the wire ends into the controller connector similar to the way you would install speaker wire.

As I stated earlier, this is actually the model I use to top off my 40-gallon sump (with RO water) that sits under my 180g reef tank.

Check the current price of the Tunze Nano Auto Top Off Osmolator 3152 on Amazon.







  • Small form factor of sensor great for small nano tanks
  • Same pump as the standard version, pump included
  • Pump fits in standard 5-gallon water jug
  • Timer safety feature to prevent pump running to long and flooding
  • Ability to set timer at 2 and 4-minute intervals (Pump .5g or 1g each time it cuts on)
  • Can be used to top off smaller sumps used on large tanks



  • If you ever need to pump more than 1g at any given time you have to reset the unit
  • Float switch is susceptible to salt creep causing it to stick if periodic cleaning is not performed.
  • Connecting pump to controller is not done with standard outlet plug

Smart ATO Micro – Automatic Top Off System

The Smart ATO Micro is a nice small form factor design.  Like the Tunze it includes everything you need right out of the box short of a reservoir and water.

The small size of the pump will fit into a standard 5-gallon jug.  The ability to just swap out 5-gallon jugs does make filling the reservoir a lot easier than having a stationary container you have to pour water into.

The water level sensor is a light-based sensor.  Move the sensor to the desired water level and magnet it to the side of the tank.  This eliminates the worry of a float switch that can stick, but replaces it with a sensor that could get covered by algae or salt creep.  No system is full proof.

The small size of the sensor though does make this a very nice option for small sumpless tanks that feed directly into the tank.  Since it is so small it can be placed in a corner.  The Blue LED unfortunately does not go off.  This may not be an issue if running actinic lighting which will already have a blue tint.

The pump that comes with this unity is capable of pumping water up to 6.5’ up into the air.  At no head height it will pump up to 74gph.

The blue light will stay on all the time.  When filling it will blink blue.  When the reservoir has run dry the pump will sense this and the sensor will blink red as well as make an audible alarm.  Once the alarm sounds the unit must be unplugged for 10 seconds to reset it.

Unit comes with a siphon break in the line to help prevent a siphon draining your reservoir and flooding your tank.

Overall this is a good unit.  It is simple to install.  It has many of the key features we like to see in an ATO system.  Price wise it sits middle of the road with comparable systems.

Check the current price of the Smart ATO Micro on Amazon.






  • Pump is included
  • Fits in a standard 5-gallon jug
  • 5 head height
  • Easy setup
  • Pump up to 74gph


  • No clear auto shutoff
  • Sensor can be covered by overgrowth reducing effectiveness

Hydor Smart Level Controller, ATO

The Smart Level Controller by Hydor is simply that.  It is an ATO controller.   That means that in the box you get the control system and the water level sensors.  The unit does not come with a pump but will work with any 110v pump US version, and 240v European version.

This makes it more versatile to fit different tank size situations but does mean there is more to buy and there for that should be included in your total cost when doing price comparisons to other ATOs that include the pump.

This also leaves it to you to properly size the needed pump, again making it a very versatile controller but giving you more to decide on.

The sensor has 6.5’ of cord allowing for placement of the controller away from the actual sensor.  The sensor has three detection level minimum, maximum, and alarm.  Each of these sensor levels is actually a thermostat.  With no moving parts or light sensors we don’t have to worry about algae growth or salt creep messing with float switches or optical sensors.  This does still leave it being impacted by wave levels and surface water flow.

The sensors compare the temperature of the water to the air temperature above the water.  The temperature of all three sensor points is compared to the temperature of the air to identify which if any of the points are under water and therefore at a different temperature than the air.

Water waves raising and falling against the sensors will impact the readings and thus its functionality.  Because of this you should shield the sensor from this or place it where there is little to no surface movement.

Check the current price of the Hydor Smart Level Controller, ATO on Amazon.








  • Versatile design can work with any pump
  • No moving parts in the sensor
  • Can be used on any size system
  • Audible alarm if water to high


  • Pump not included
  • Sensor impacted by water waves

JBJ Automatic Top Off

The ATO by JBJ has been around in the hobby for many years and is a time tested controller.  I say controller because it does not come with a pump in the box which groups it with the Hydor.

This does make it a versatile option allowing you to pair it with any pump you choose giving you the ability to set this up on any tank size with the right pump.  Again this versatility comes with the need for you to figure out which pump you need.  This could be more than someone new to ATOs wants to tackle.

This unit is feature packed though.  It has two float switch sensors which means it is susceptible to issues caused by salt creep or algae growth so periodic cleaning is recommended.  With regular cleaning there shouldn’t be any issues.

The issue offers two modes.  In mode A you place one float switch in your sump/tank and the other in your reservoir.  The float in your sump/tank will activate the pump when it reaches its bottom point and cut it off when it gets to its top floating point.  The second float is mounted low in the reservoir and cuts off the pump if the water level drops to low so it does not run dry.

In mode B both sensors are placed in the sump/tank and sensor 1 acts as the low water point and sensor 2 the top water point.  The pump cuts on with sensor one and off with sensor 2 giving you a larger water fluctuation.  With a reef tank this mode may not be the best suited.

In mode A though you can flip the second sensor in the sump and flip it upside down.  This puts it in position to act as a high level sensor and cut off the pump if the water gets to high rather than monitoring your reservoir.

You can also flip the floats inside the sensor by removing the clip flipping the sensor over and replacing the clip.  This serves the same purpose as flipping the whole float upside down.

Inside the controller there are two handy adjustments.  First is the max time the pump stays on for.  Factory settings is around 3.5 minutes.  The dial can be turned left with a screwdriver all the way up to about 14 minutes.  There are markers indicating 3, 5, and 7 minute marks.

The second adjustment is an on off switch which can be set to on causing an audible alarm to sound each time the pump activates.  This may not be desirable in most cases but it is there for those situations where that is needed.  Maybe you are trying to figure out how often the pump is cutting on or how long it stays on for example.

Overall the JBJ has some great features and gives you versatility if that is what you need.  For us tech heads to like to tweak and play and tinker this is an awesome ATO giving us the ability to get things just right.  If you want an out of the box plug and play option I recommend Tunze.

Check the current price of the JBJ ATO on Amazon.









  • Versatile, works with any pump
  • Multiple configuration settings
  • Adjustable pump shutoff timer


  • Pump not included
  • Float switches require occasional cleaning
  • Does not include magnetic mounting

Innovative Marines Hydro Fill Ti ATO

The Hydro Fill Ti ATO by Innovative Marines made my list not just because of the ATO controller but because of the ATO pump they offer as well.  The biggest thing to point out here is that these are sold as separate products but work great together.

The fact that they are sold separate could both be a pro and a con.  Fist lets cover the con.  If you didn’t know what you were buying you wouldn’t know to grab them both or of the total cost to operate.

The pro is that, if you already have an ATO controller or want the pump but prefer a different controller like the JBJ or Hydor; the Hydrofill pump would work great with those too. 

I would love to see a combo pack though that was a little cheaper than the two separately, just saying.

The controller comes with two sensors that use conductivity to determine water level.  This means that they are operating like a TDS pen.  What is that?  That is a measure of the total dissolved solids.  This is measured by how well the water conducts electricity.

TDS pens are used to compare how crappy your faucet water is vs. how clean RODI water is.  Since RODI water is so clean it does not conduct electricity but your salt water tank sure will.

Mount the two sensors using the magnet mounts so that the metal tips point down at the desired low and high water range.

What I really love though is the Hydrofill pump.  It is an external diaphragm pump which was specifically designed to operate as an ATO pump.  It has a built in check valve, is self-priming, and run dry protection.

For a small extra fee Innovative Marine also offers a connector bracket system that mounts to both rimed and rimless thanks and has a barbed connector to hook a hose from the pump to it giving a secure clean mounting solution.

The down side to this pump is that it only has a max head height of 5’ and a max output of 15gph.  Because of this if you are feeding your tank directly w/no sump 5’ may not work.  Also at 15gph it is only half as powerful as the Tunze pump.

This does not mean it is inferior just that it would have to run twice as long each time it kicks in and may not be able to handle some of the larger tanks that need larger volumes of water pumped in faster to maintain their level without the pump running very frequently.

Check the current price of the Hydro FIll Ti ATO on Amazon.

Check the current price of the Hydro FIll Ti Pump on Amazon.









  • Versatile, works with any pump
  • Sensor has no moving parts
  • Uses conductivity which is unique amongst its competition


  • Need to buy pump separate
  • Pump only has 5’ head height
  • Not suited for larger tanks
  • No backup failsafe

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My personal choice is going to be the Tunze either the 3155 or 3152 Nano depending on your needs.  Hydor and JBJ give great flexibility and for some systems that may be needed but for a straight forward plug and play ATO option, you can’t beat Tunze.

Any way you slice it though any ATO is better than no ATO. The benefits in convenience alone should make any aquarium owner want one.  Then match in the health benefits for your critters and you should planning to buy one.

Are there any ATO Systems that didn’t make the list and you think they should?  Let us know below.







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