Osmolator Universal 3155 Auto Top Off - Tunze

Tunze Osmolator Universal 3155 Auto Top Off
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  • Tunze Osmolator 3155

Osmolator Universal 3155 Auto Top Off - Tunze

97% Recommend this to a friend (269 of 277)

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
Will the sensor fit in the sump of an innovative marine 30L ?
Question by: Ian on Feb 9, 2016 8:33:00 AM
Great question!
This will work great in the back of a nuvo 30! Feel free to let us know if you have any further questions!
Answer by: Connor (Admin) on Feb 9, 2016 6:11:00 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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  • the best By bigjay on 1/21/2016

    had my reef tank 2 year ben top off the water my self the tune osmolator rocks keep my sump at the prefect level it the best buy it best thing i got for my tank
  • The Mercedes of ATOs By jsbull on 1/2/2016

    This thing is awesome. Redundant shutoffs, quiet operation, peace of mind. I highly recommend it.
  • The Mercedes of ATOs By jsbull on 1/2/2016

    This thing is awesome. Redundant shutoffs, quiet operation, peace of mind. I highly recommend it.
  • Great product By timothy on 12/27/2015

    Very easy to setup. Very accurate. Never have to worry about topping off
  • does what it does By sadt3arz on 12/26/2015

    Awesome product, easy to setup and works perfectly and flawlessly. Highly recommend. Only reason i didnt give 5 star is because the pumps kind of loud. My tank s in my room and at night when sleeping i can hear it turn on and off everytime it refills. I wish it had like a night mode or something. Be careful with the placement of the line so you dont end up syphoning ur reservoir. I learned that the first day.
  • Great product By Mike on 12/18/2015

    This is my third (sort of) osmolator. Some years ago I bought a used osmolator after researching what seemed to be considered the "best". I was particularly captivated by the optical sensor. Over the next few years I had to replace a few parts, but as my unit was used I felt that that was fair. My two biggest issues were that if the water ever dropped too far the controller would time out, or overheat or something and never restart. The second issue was the noise that the pump made. I even replaced my old one and the new one was louder. When the ATO became totally unreliable I decided to research what to replace it with, thinking that I would NOT pick another tunze because of the above issues. However, research again led me to the Tunze so I took a chance on a great Black Friday sale and have been running my new osmolator for a month now. I LOVE it. Reliable and quiet - who could ask for anything more.
  • Pretty much perfect By Olliesshop on 12/17/2015

    After trying other types and brands of ATOs, I finally decided to go with the Tunze 3155. I love that I can depend on it to keep the water level at a fairly precise level. The only thing that bothers me a little is the pump turns on every time the ATO is turned off and on. So if I have to cycle it off an on it will result in a the water level being too high and overdosing some on my Kalkwasser. This is something I'm adjusting to, though.

    Overall, this is a great product and I would highly recommend it.
  • Great Product - Great Features - Sleep GOod at Night! By Steve D on 12/16/2015

    I have had this setup for about 2 weeks now and my lower back is jut loving it! I used the separate mounting magnet for the safety / overflow sensor as I placed mine in a 45 gallon sump in the return chamber. I had done a lot of research and looked at countless reviews and this unit always came out ahead of the rest. It is a bit more $$ than some of the others but it is well worth it. I don't miss any sleep wondering if my sump will flood in the middle of the night as I have tested the overflow sensor and alarms. Unit works very well. The BRS videos were very helpful in getting this setup, explaining all the parts and features. It only took about 15 minutes to get everything in place and the little pump actually fits nicely inside of my 5 gallon containers. Great Job Tunze & BRS!!!!
  • Second to none! By John on 12/16/2015

    This is simply the best ATO one can buy. Extremely easy to set up and with several mounting options, you can't go wrong. Keeping your salinity stable is no longer problem.
  • Can't beat the unity; Can't beat the track record! By Steve on 12/13/2015

    Great ATO! Love the optical eye. Easy set up and works flawlessly. With all the modification and accessories possible, it simply can't be beat.
  • Top of the line By Fred on 12/11/2015

    This is a great ATO! Reliable and compact. Worth the price! Why buy something you're going to just wind up replacing with this eventually?? Do it right the first time!
  • Been running solid for 3+ years By Great product on 12/9/2015

    The original one I ordered appeared to have quit working, so I ordered a new one. The only thing wrong was the power converter cord. I replaced it and the unit works fine.

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