Tunze Osmolator Universal 3155 Auto Top Off

Tunze Osmolator Universal 3155 Auto Top Off
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  • Tunze Osmolator Universal 3155 Auto Top Off
  • Tunze Osmolator 3155

Tunze Osmolator Universal 3155 Auto Top Off

97% Recommend this to a friend (246 of 254)

SKU: 202307
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Quick Overview

Why does BRS recommend this?

Bulk Reef Supply tried many different water level switches and auto top off solutions before giving the Tunze 3155 Osmolater Universal Auto Top Off BRS Recommended status. Despite its higher price this is by far our favorite solution. Spending the extra money on a Tunze Osmolater will give you that extra peace of mind and insurance from having a disaster that ruins carpets or hard wood floors.  For the value, and reliability it is hard to beat.


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Product Questions

I have a litermeter 3 that has failed. It pushes water about 30 feet from the litermeter to a sump tank. Will this pump do the same?
Question by: Spencer on Apr 29, 2014 12:54:00 PM
Hi Spencer,
The max head pressure on the included pump is about 7ft. It likely wouldn't be well suited to this application. That said though, you can get the additional switch socket adapter which would allow you to control any pump via the osmolator system. That way you can use a pump that can handle the head pressure.
Answer by: Bulk Reef Supply on Apr 29, 2014 12:54:00 PM
If i have a wave maker and this ATO in the same tank (I don't have a sump), would the waves cause the pump to turn on and off constantly? Thanks
Question by: Christian on Jul 17, 2014 9:40:00 PM
Hi Christian,
In theory it could present a problem but the circuitry in the Osmolator should help to prevent that. Every time the ATO is triggered it runs for a minimum of 15 seconds. It adds overfill to prevent short cycling. In most cases this should be sufficient unless the height of your waves is constantly changing.
Answer by: Bulk Reef Supply on Jul 17, 2014 9:40:00 PM
How much head will the pump push
Question by: Jeff on Nov 28, 2013 2:50:00 PM
Hey Jeff,
The pump on the osmolator has a max head height of 7ft.
Answer by: Bulk Reef Supply on Nov 28, 2013 2:50:00 PM
What is the measurement on the pump, as in the widest part? I need to know if it will fit in my container I plan to buy.
Question by: David Price on Jul 7, 2015 12:10:00 PM
Hey David,
The diameter of the pump is just under 2" this is how wide the cap will need to be in order to allow the pump to pass through.
Answer by: Connor (Admin) on Jul 7, 2015 1:03:00 PM
Will this work with the Apex controller?
Question by: Jim on Aug 3, 2014 11:31:00 AM
Hi Jim,
The Tunze Osmolator is an entirely self sufficient system. It is perfectly fine to connect the Osmolator to one of the Apex's power outlets incase you wanted to cut the power for some reason (like when you are doing a water change) though the Apex isn't capable of reading the Osmolator's sensors or anything of that sort.
Answer by: Bulk Reef Supply on Aug 3, 2014 11:31:00 AM
Would you happen to know the max head pressure of the Tunze Osmolator's pump?
Question by: David on May 10, 2014 11:50:00 PM
Hi David,
The max head pressure on the osmolator pump is 7ft.
Answer by: Bulk Reef Supply on May 10, 2014 11:50:00 PM
what pump do you suggest to go with the3155 osmolator?
Question by: BILL on Nov 13, 2013 7:37:00 AM
Hey Bill,
The Osmolator actually comes with the pump right in the package so you don't need to buy one!
Answer by: Bulk Reef Supply on Nov 13, 2013 7:37:00 AM
Just wondering if I need 2 ATO. I know you can place it either in the sump or in the tank, but my question is if I place it in the sump will it also refill my tank in that process. I don't see that happening but if it does please let me know.
Question by: Ronald on Jan 13, 2014 2:32:00 AM
Hi Ronald,
If you have a sump, it MUST go in the sump, in the chamber where your return pump is. This is the only area of your tank that will have a changing water level depending on evaporation (unless its far too late!). Only one ATO would be necessary but it must go in that location if you have a sump.
Answer by: Bulk Reef Supply on Jan 13, 2014 2:32:00 AM
My Tunze Osmolator ATO keeps getting stuck in the ON position. I soaked the electronic eye in vinegar and it was fine for a week, but then it stuck on again. Do these electronic eyes fail after many years of use? I've been using it for about 4 years. Is there any way to replace the eye without having to buy a whole new system?

Daire Fontaine
Question by: Daire Fontaine on Jan 17, 2014 12:23:00 AM
Hi Daire,
We run into very few issues with these but after that length of time I would imagine its possible for any component to fail. We don't have individual parts like the optical eye, but if you give Roger and company a shout over at Tunze USA, he might. At the very least he could likely help you narrow down exactly what it is. The number I have for them is 512-833-7546 tunze@sbcglobal.net
Answer by: Bulk Reef Supply on Jan 17, 2014 12:23:00 AM
Does the pump get placed directly in the top off water reservoir? If so what are the dimensions of that pump? I am trying to decide what kind of reservoir I can use before purchasing a ATO.
Question by: Mark on Jun 5, 2014 8:39:00 AM
Hey Mark,
That is correct. Its a submersible pump that gets stuck right into the top off water. If your interested in this ATO system you should give this blog article a quick read. It goes into a lot more detail on the system (which is one of my favorites).


Answer by: Bulk Reef Supply on Jun 5, 2014 8:39:00 AM

Full Details

The Tunze Osmolator 3155 is one of the safest ATO’s available. It utilizes an electronic eye sensor to monitor water levels and a backup float switch sensor with an optional audible alarm. The controller protects the quiet metering pump from running dry and reports an empty storage tank. Two magnetic holders for sensors and multiple mounting brackets included as well as a pump.

LED indicator lights the Osmolator will allow you to visually see if their are any issues with your top off system. 

Two technologies of water level monitoring, and built in safety back-ups.

The optical eye sensor will keep the water level very steady to keep the salinity from fluctuating in your aquarium.  The Osmolator does work well with kalkwasser.  



Note: With any pump, kalkwasser can cause premature wear.  New pumps are available at a very reasonable price. 

Replacement Metering Pump



Click Here



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  • Piece of mind, stable salinity By Adam on 9/15/2015

    Wow, what a great purchase. This is the way to keep your tank topped off for evaporation loss and keep salinity EXTREMELY stable. Drop dead simple with two forms of measuring water needed. Takes about 5 minutes to install and get running. This is especially helpful when I travel and have a sitter care for the tank. All they have to do for water is keep the top off jug full and everything else from a water level perspective is covered. I really love this product. This is the best solution for automated top off I have seen.
  • Love it By Jorsreeftank on 9/6/2015

    This is my third ATO system so I feel qualified to Say this is the best one. It's easy to install an what I like best is its safety redundancy and the fact that it really keeps my water where I wantbit!! It kicks the pump on as soon as the level drops. I would say less than a pint
  • Best ATO By Joesreeftank on 8/23/2015

    Do not screw around with the others, This one does what it says, It keeps my Sump right were I set it, I have a 40 gal sump and if it looses less than a couple of pints it kicks on and tops off..Easy to install
  • Must have item By Jim on 8/11/2015

    Great auto top of package, very easy to install and get set up. Pump is a little loud for how small it is (only hear it when in the room, which i like since I know it is working). The price is a little high, but totally worth it once you see it in person and have it setup. Would definitely buy again.
  • It's a Must Have By Derek on 8/10/2015

    Hands down the best ATO, I have every owned and I can't see myself using anything other than this product. Wrongs flawlessly and most definitely it put me at ease not having to worry about adding water or flooding my sump.
  • Excellent ATO System By 40g Long on 7/22/2015

    After we finally got our tank set-up, I was amazed at the amount of evaporation from day to day. Being new to the hobby, I was concerned with maintaining stable water parameters. . I watched all the videos, researched the high tech models to the DIY versions, and decided the Tunze would be the ATO for our system. Since it's installation it has been performed flawless. I highly recommend this product.
  • great product By rrosenb on 7/22/2015

    So easy to install and works perfectly. I wish all aquarium products could be more like this
  • Excellent ATO By Conny on 7/22/2015

    Well built, easy setup, reliable- excellent ATO
  • Works great By Dave on 7/17/2015

    This thing works great!! A little pricy compared to other top off brands but very dependable and unlikely to fail. I would recommend this product over all the other ones on the market. Easy to install too!
  • Solid and reliable By Steve on 7/16/2015

    The Tunze 3155 Auto Top Off system is the best there is. This is my second 3155 system, just installed on my new 185 gal tank build. My first 3155 system (on my first,180 gal, tank build) continues to function perfectly, with no issues, after more than 3 years of service.
  • Nice simple and reliable By JEMM55 on 7/14/2015

    Better than the rest...the electric eye beats the mechanicals hands down
  • Loving my Tunze ATO 3155 By A.J. on 7/13/2015

    After a fair amount of research and reading many customer reviews I pulled the trigger and bought the 3155. Works perfect and only took a few min to set up. It really take the stress out of traveling, not having to worry about your water level. ALSO - has kept my salinity rock solid for over 3 months now, which my corals are loving!!

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