Tunze Osmolator Review: The 3155 And Nano 3152 Auto Top Off Systems

Should you grab a Tunze Osmolator?

In this Tunze Osmolator review, you will see that this product is not a necessity for your aquarium.

However, those who do get one find it is such a handy and convenient addition that they wouldn’t be without it.

Among its many benefits, a big PLUS is that it helps maintain a safe and healthy aquatic environment – and who doesn’t want that?!

Tunze Automatic Top-Off System

Tunze 3155 Osmolator

  • The top recommended auto top up system
  • Low voltage pump, mounting magnets, tubing controller and power supply
  • Extremely reliable: multiple redundancy systems
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Tunze 3152 Nano Osmolator

  • For smaller tanks under 55-gallons
  • 11V power supply, metering pump, 118" hose, magnet holder, sensor cover
  • Also exceptionally reliable
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Tunze Reservoir

  • 7.1-gallon reservoir
  • Easy to clean
  • Ultimately, it's just a bucket
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Tunze is a reliable name in the world of aquatic equipment and environments, and what’s more, they actually invented the Osmolator!

Other names for this device include:

  • Automatic Top Off System (ATO)
  • Osmotic Regulator
  • Water level regulator

For the best run-down on what the device does and who it’s good for, you must watch this summary video.

What’s an osmolator?

An osmolator is an automatic top-off (ATO) system.

It adds new water to compensate for the evaporation of water and can be used with freshwater or saltwater aquariums, including reef tanks.

It makes the refilling process much easier and less of a hassle.

What are osmolators used for?

Water evaporates. It’s a fact of life.

Fish keepers wanted something that could protect their tanks, fish, and aquatic plants. As the water level drops, a small pump is triggered and starts the refilling process.

Osmolators are an invaluable part of saltwater and freshwater tanks. They help maintain and stabilize the salinity levels.

They also stabilize the filtration system to prevent your tank from drying out or becoming cloudy.

Who needs an osmolator?

Anyone who has a fish tank or an aquarium system can greatly benefit from an osmolator.

They ease the burden of having to think about water levels or your tank drying up.

They’re also important if you’re frequently away from your home. Or if you’re going out of town for a week or more.

Alongside a system for feeding your fish while on holiday, you can have a stress-free break.

1. Tunze Osmolator Universal 3155


tunze osmolator 3155

The Tunze Osmolator Universal 3155 comes with everything you need to keep your aquarium in tip-top condition.

It is made with high-quality, durable components that are hard to beat.

It weighs less than 3 pounds and measures 4 x 1 2.5 inches.

Included with the osmolator are a power supply unit, hose, metering pump, and sensor holding device. All you need is a reservoir tank with prepared water.

The metering pump on the Tunze Osmolator 3155 is submersible and low-voltage.

The tubing can reach up to 6 feet long. The long tubing makes it versatile and boosts its functionality. It’s one of the best features of this osmolator.

The 3155 osmolator features an electronic optical eye sensor. This is the main sensor. It gives reliable readings without being affected by water movement or waves in the tank.

This osmolator’s back-up sensor is the manual floating sensor. It comes with an audible alarm, which sounds when the floating sensor is triggered.

The alarm makes it easier for you to keep track of the osmolator’s activities. It also ensures that the entire system is working properly.


The Tunze ATO also features an automatic shut-off system.

It triggers the pump to shut down if it keeps working for more than 10 minutes. It also has a minimum run time of 20 seconds.

The auto shut-off reduces the risk of the pump over-filling the tank. It also protects it against dry-runs, which can damage the pump and the tank itself, as well as harm your fish.

The kit features several LED indicator lights. They offer a clear display of the status of both sensors. They also light up when the pump is working.

The Tunze Osmolator kit comes with 2 durable magnetic sensor mounts.

You can use one or two of the mounts for easy set-up. The magnetic mounts are used to install the kit onto any aquarium or reef tank. just make sure the walls are ¾ of an inch or less.

There are also Velcro strips that come with this kit. They allow you to attach the level controller to the inner walls of the tank stand.


  • Backup sensor
  • The floating sensor uses an audible alarm
  • LED status lights
  • Auto shut-off
  • Compact
  • Lightweight


  • The pump is loud
  • It is relatively expensive

2. Tunze Osmolator Calcium Dispenser

The Tunze Osmolator Calcium Dispenser is placed between the feed hose and the metering pump.

It helps supply minerals and nutrients directly into the fish tank.

The minerals are a blend of water and calcium hydroxide, which is also known as kalkwasser. It’s set to pump in calcium at regular intervals and in small amounts.

Dosing the water with these minerals ensures that the tank water maintains a stable pH value. This Tunze calcium dispenser works great with aquariums up to 200 gallons.

Its dimensions are 18 x 5.5 inches. It comes with a calcium hydroxide fill.

You’ll also find mounting clips to attach it to the tank, as well as connectors that attach it to the osmolator.


  • Easy to set up
  • Maintains stable pH balance
  • Works with all types of aquariums
  • Moderately-priced


  • Mounting clips aren’t durable

3. Osmolator Reservoir

tunze ato

When you buy the Tunze Osmolator Universal 3155, you get everything for an easy ATO system set up.

The one thing that’s missing, however, is the reservoir and that’s where the Tunze Osmolator Reservoir comes in.

Its large capacity is ideal for tanks up to 150 gallons in size. This 7.1-gallon reservoir features a wide cover opening.

It makes cleaning and maintenance simple and hassle-free.


  • Large capacity
  • Wide opening for easy access


  • Relatively expensive

4. Tunze Osmolator 3155 With Reservoir

tunze 3155

The Tunze Osmolator with a 7.1-gallon reservoir makes a great addition to your aquarium.

The ATO kit comes with everything you need to get your aquarium ready.

The reservoir features a large cover opening. It makes refilling and cleaning hassle-free.

Its size is 13.5 x 13 x 17 inches and weighs 2 pounds.

The size of this reservoir is perfect for large tanks that are under 100 gallons. One fill will last for approximately one week.


  • Includes large-capacity reservoir
  • Great for large aquariums
  • Backup sensor
  • Auto shut-off
  • LED indicator lights


  • Not quite the complete kit

5. Osmolator With Calc Dispenser And Reservoir

tunze auto top off

This complete kit has everything you need for a fully-functioning ATO system.

The osmolator uses an optical sensor, as well as a backup floating switch. There’s also the fail-safe auto shut-off function.

It’s easy to install with the help of 2 magnetic mounts. The LED indicator lights make it easy to use.

The calcium dispenser feeds the right amount of kalkwasser into your aquarium. It helps stabilize the salinity and pH value of the water.

The 7.1-gallon reservoir is the perfect size for all types of tanks. Its wide opening makes it easy to clean and refill.


  • Comes with all components
  • The large-capacity reservoir features a wide opening for easy access
  • The calcium dispenser maintains stable pH levels
  • Works great with large aquariums


  • The water pump is loud
  • The magnetic holder on the calcium dispenser is flimsy

6. Replacement Metering Pump

automatic aquarium water top up

This Tunze Osmolator replacement metering pump is low-voltage.

It stands at 2.5 x 2.5 x 4 and weighs a little over 3 ounces.

The good news is that taking out the old one and replacing it with this new pump is quick and easy.

The one thing some users complained of was its relatively loud pumping noise.


  • Durable
  • Easy to install
  • Cost-effective


  • A little noisy

7. Tunze Osmolator Nano 3125

Tunze Osmolator Nano 3152

The Tunze Osmolator Nano 3125 is a great option if you have a small aquarium under 55 gallons.

It features one manual sensor, which provides a processor-based anti-overfill system. It shuts down the pump after a full 5 minutes.

This ATO kit comes with:

  • Low-voltage metering pump
  • Power supply
  • Float switch

It also comes with one magnetic holder, 3 meters of tubing, and a protective shield.


The Nano 3125 can be installed with the help of a magnetic holder.

You can put it directly on the edge of the aquarium or in a cabinet filter.

It’s best used with aquarium walls up to 0.39 inches thick.


  • Uses anti-overfill system
  • Great for small aquariums
  • Durable design


  • Magnet holder is too big for rimmed aquariums
  • Needs constant resetting
  • A bit pricey

8. Nano Osmolator With Reservoir

best osmolator

This kit includes all components of a fail-safe, low-maintenance ATO system.

It’s one of the best osmolators for small aquariums with a capacity of up to 55 gallons.

It features one float switch sensor, 3 meters of tubing, and a metering pump. The magnet holder attaches the osmolator on the aquarium edge or in a cabinet filter.

Its protection system guarantees that the water will always be at the proper level. This osmolator kit also comes with its own reservoir.

What could be more perfect?


  • Anti-overfill system
  • Works great with small aquariums up to 55 gallons
  • Comes with a large-capacity reservoir


  • Needs constant resetting
  • The magnet holder is too big for rimmed aquariums
  • The price is a bit high

Top Off System Features

tunze auto top off system

Below are several features you should look out for when buying a proper ATO system for your fish tank.

1. Redundancy

This is one of the most important features of an osmolator.

ATO systems come in a wide range of redundancy features.

Some use one sensor, others use a dual-sensor combination. There are also timers and automatic shut-off features.

They all work to ensure that the water remains at the proper level at all times. Either having too much or too little can prove harmful to your fish and aquatic plants.

2. Sensors

Osmolator is short for osmotic regulator. Osmolators are used to measure the water level.

This can be done with a manual sensor or an optical sensor.

A good osmolator will have both types of sensors working together.

Whichever type of sensor you decide to go with, make sure you clean it at least once a month. Keep an eye on it to ensure it’s running smoothly.

Manual Sensor

Manual sensors resemble the float switches found in toilet tanks.

This type is the most common because it’s easy to set up and quite reliable to maintain.

It works via a switch that floats on the surface of the water.

It keeps pumping water into the tank until the water level rises to a certain level. That’s when the switch gets triggered. Then, it signals the ATO system to shut down.

Though quite straightforward, it does come with a few drawbacks.

For one, it can get clogged with salt, tiny critters, and debris. It can also get damaged as it floats at the water surface.

Optical Sensor

The electronic eye sensor keeps an eye on the aquarium’s water level.

When it rises above where the sensor is set, it signals the ATO to stop pumping water.

This is a reliable piece of equipment, mainly because it has no moving parts, unlike the manual sensor.

However, it’s prone to get clogged up by impurities and debris over time.

Temperature-Based Sensor

Temperature-based sensors are similar to the optical eye in that they don’t have any moving parts.

Yet, the best thing about this type of sensor is that they are smaller than the other types. This makes it perfect if you’re working with a small space.

The probe is put into the water. This cools off the sensor. When the water drops below the level of the probe, it heats up.

As a result, the pump is activated, and the water rises once again.

An aquarium chiller can usually be used in tandem with an ATO without any trouble.

Conductivity Sensor

This fourth, and final, type of sensor also doesn’t rely on moving parts to measure the water level.

They work with the help of conductivity probes, which scan the tank surface.

They use this to detect whether or not there’s water between each of the probes. When they’re not aware of any water, they signal the pump, and the water level starts rising again.

3. Auto Shut-Off Timers

This feature can come in real handy if the main switch ever fails for any reason.

The automatic shut-off timer can come in and turn off the pump before a real water emergency happens.

It can save your tank from overflowing, and your fish from flowing out of the tank along with the extra water. It can also save your home from flooding.

Tunze Auto Top Off System

In our Tunze Osmolator review, we discussed the importance of ATO systems.

They’re a great investment if you want to keep your aquarium system in optimal condition.

One of the best osmolators on the market, the Tunze Osmolator Universal 3155 is extremely well-built.

It’s reliable and comes with many great features that will help protect your aquarium for years to come.

Next up, check out these aquarium overflow boxes – there’s no end to the different set-ups you can integrate into your aquarium!

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