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Tunze Osmolator 3155

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Original price $359.95 - Original price $359.95
Original price
$359.95 - $359.95
Current price $359.95

The TUNZE Osmolator 3155 is an automatic top off (ATO) system for marine and reef aquariums. The Osmolator – short for Osmotic Regulator – automatically adds freshwater to compensate for evaporation in your saltwater aquarium.

The high-tech German designed Osmolator uses a precision optical sensor to measure the water level. Water continuously evaporates from the aquarium, leaving the salts behind. This causes the salinity to increase. When you manually pour in water the salinity is suddenly decreased. This “salinity rollercoaster” stresses the fish and invertebrates by causing extreme fluctuations in osmotic pressure.

The Osmolator puts an end to this by continuously sensing the rate of evaporation (water level) and adding back the right amount of freshwater. This brings osmotic stability to the aquarium and keeps your sump and aquarium water at the right level.

Here’s what we really like about the Osmolator:

  • TUNZE invented the ATO and has been perfecting it for 30 years
  • Precision optical water sensor detects the slightest change in water level
  • Easy to install and adjust using the Magnetic Sensor Mount
  • Status lights display functions and sounds a warning in case of trouble
  • Built-in safety features prevent overfilling of the aquarium and shuts the system down
  • Comes ready to install – all you need is a reservoir or bucket

The Osmolator can be used on any aquarium, even freshwater tanks. It works when installed inside the aquarium on tanks without a sump. If you have a sump the sensor must be installed there since that is where the water level changes as it evaporates.


  • Complete kit with power supply, metering pump, hose and sensor mounting system.
  • The water level sensor has no moving parts to wear out and is not affected by suspended dirt, light or magnetic fields.
  • Optical sensor ignores gentle water movements caused by wave makers and circulation pumps.
  • An independent safety sensor with a separate power supply circuit and relay ensures reliable overflow protection.
  • A microprocessor monitors the water level in the reservoir and prevents the pump from running dry.
  • All functions are clearly displayed by colored LEDs and warning signals.
  • The Osmolator uses ultra-low voltage for complete safety.

For smaller aquariums, check out the Tunze Nano Osmolator 3152. If you need a container to hold your top off water, the Tunze Osmolator Storage Container 5002.25 is an ideal choice.


  • Metering pump flow rate based on pumping height: 3 feet:13.7gph, 5 feet:12.4gph, 6.5 feet: 7gph, Maximum pumping height: 7.22 feet
  • Magnetic Mount System: Maximum glass thickness ½ inch
  • For indoor use only
  • 2-year warranty

Product Manuals & Documentation

Tunze Osmolator 3155 ATO Auto Top Off System

Owner's Manual available for download in Adobe PDF file format

Manufacturer Info

For forty-five years the name TUNZE® has been synonymous with aquariums. Many products and components which we have developed in the course of time are now standard features of aquarium technology today. This confidence in TUNZE is no coincidence. The company has remained true to its philosophy right from the beginning, researching water biotopes all over the world to develop components which enable natural water conditions in the closed environment of an aquarium. In the implementation of know-how, effectiveness and efficiency, TUNZE® takes the lead in environmental compatibility. Their motto is: quality and reliability. These values are upheld with well thought-out and precise tool making by highly qualified experts. The brand names of TUNZE® and Turbelle® meet the expectations of the seal of quality; Made in Germany. Many of the products shown in this catalogue are protected by national and international patents.


Manufacturer`s Warranty: 2 years from date of purchase.