Osmolator Universal 3155 Auto Top Off - Tunze

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

Osmolator Universal 3155 Auto Top Off - Tunze

95% Recommend this to a friend (301 of 316)

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.

Max Head Height: 7.2 ft


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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
Hi, by head pressure does this mean how many feet the pump will push the water? So if my pump is in my cabinet under my tank will it push the water to the top of my tank? Thank all over at brs..
Question by: ray eutermarks on Apr 9, 2016 11:06:00 AM
Great question!
Head pressure is the maximum height that the pump can push water vertically. If your tank is higher than the Head height listed this will not be able to work. Feel free to let us know if you have any further questions!
Answer by: Connor (Admin) on Apr 13, 2016 6:29:00 PM
You say additional switch socket adapter to handle more head pressure. Where can i find this? I traveling about 15 feet 9 of that is up.
Question by: Kelvin J Wiese on Jan 4, 2017 10:41:00 AM
Great question! The Switched Socket Outlet will allow you to use any pump of your choice, rather than the pump that comes with the 3155. You simply choose a pump that can handle those head heights, plug it into the Switched Socket Outlet, and the Tunze sensors now control that outlet with that pump. Below is a link to that Socket, that includes a demonstration video in the stock photos on that page. :-)


Answer by: Randy (Admin) on Jan 11, 2017 11:30:00 AM
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
What are the dimensions of the blue Tunze controller module/box. I am trying to determine if this will fit on an existing mounting board. Also, how does it mount? Adhesive or punchouts in the back plate. I don't see screw holes like the Apex modules have. Thank you.
Question by: Elisabeth Duarte on Mar 21, 2016 1:55:00 AM
Great question!
This will be 4" long, 2.5" wide and 1" thick. it will require a Velcro pad to stick or to be used with double sided tape.
Answer by: Connor (Admin) on Mar 21, 2016 6:50:00 PM
I have a 10 gallon tank and am concerned with salinity swings. I top off my tank every day but I'm afraid most of these ATO would not fill the evaporated water before my salinity starts to rise. What is distance vertically from the low to full position?
Question by: Andrew on Apr 15, 2016 10:31:00 AM
Great question!
The optical eye sensor will usually sense and fill within the range of 1/4" Feel free to let us know if you have any further questions!
Answer by: Connor (Admin) on Apr 19, 2016 4:05:00 PM
I have turned the control up to the max however it will not keep up with the evaporation? my tank is a 120 gal with 35 gal sump.
Question by: Joe Ojeda on Apr 3, 2016 9:25:00 AM
Great question,
If you have calibrated the pump to maximum output and it is still unable to keep up you will want to ensure the pump is filling the proper amount. It may be that the head height (the vertical height the pump has to push the water) could be too high or other pump related problems. We have used the osmolator on larger systems with quite heavy evaporation with great luck. If the issues continue you will want to contact Tunze directly to see if the pump is working correctly! I have included their contact information below!
Tunze – 512-833-7546 tunze@sbcglobal.net
Answer by: Connor (Admin) on Apr 6, 2016 11:32:00 AM
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
Hello. I bought a used Tunze osmolator today. Do you sell replacement parts, such as the magnetic sensor holder. And, what is the best way to clean the sensors? Thank you. :)
Question by: Eli on Dec 30, 2016 4:16:00 PM
Great question! We only carry the replacement pumps for these systems. You may need to reach out to Tunze directly to verify whether those parts are available separately. We have found that the best way to clean these sensors is with 50/50 vinegar and water, or 5% citric acid. :-)
Answer by: Randy (Admin) on Jan 9, 2017 9:18:00 PM
Could you please post or email me, what differs from the 3152 and the 3155?
Question by: Daniel on Mar 22, 2016 1:37:00 PM
Great question!
The 3155 will have 2 sensors the mechanical float and the optical eye. The 3152 will only come with the mechanical float switch. Feel free to let us know if you have any further questions!
Answer by: Connor (Admin) on Mar 22, 2016 2:08:00 PM
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
I purchased the osmolator from your site and was under the impression it had an alarm that went off if the ATO container was empty to prevent the pump from burning up. I've tested this, but my pump just keeps running and there is no alarm?
Question by: Joel Sabin on May 3, 2016 10:01:00 AM
Great question!
This osmolator will run for typically 10-15 minutes in a dry container. eventually the controller will sense that it has not filled in a timely manner and should turn the system off. if this continues to not be filled there may be red flashing on the controller. For direct assistance you will want to contact Tunze directly!
Answer by: Connor (Admin) on May 3, 2016 5:01:00 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.  


Max Head Height: 7.2 ft


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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  • Works Great By Dave on 4/20/2017

    Easy to use and setup, works great!
  • EASY INSTALL WORK LIKE A CHARM By Red on 4/13/2017

    I got this one in from BRS and nice easy install and works like a charm. just be careful of the magnets you get those stuck they are a pain to pull apart. tip leave them on the foam until you need to put on tank.
  • Great product By Flea on 4/4/2017

    Works great keeps water lol steady
  • It works By Brian on 4/3/2017

    It's impossible to say whether this is a quality product after only owning it for a month, it's brand new so I would fully expect it ro work and do everything as advertised. I'll just have to wait and see how it functions 2 or 3 years from now. I can say that it was easy to setup and has functioned properly so far. I purchased this ATO really just for times when I'm away from home and can't manually top off my tank. I prefer to manaually fill the tank each morning after I test the salinity. I'm going to have to either fill the ATO reservior or the tank, may as well just fill the tank. I bought this unit because it was the most expensive and had the best reviews. Time will tell if it was worth the investment.
  • Reliable! By Leo on 3/27/2017

    I bought this ATO about a year ago...no problems and has worked flawlessly ever since. After I got it, I wondered why the heck I waited so long to get it.
  • Great Addition By Mike on 3/15/2017

    This is a must have product. I have a 3155 and 3152. They took a little effort to dial in but once set they have worked flawless. I no longer worry about checking daily water levels. I fill the reservoir once a week and let the system do the rest.
  • It Works Now By RobertP on 3/14/2017

    Read and heard many good things about this ATO prior to purchasing. Hooked it all up and got my sump level where I wanted and it woke me up the first night screaming about too much water. I check it and the ATO DUMPED TOO MUCH WATER! WTH! I unplugged it and left it for the day. Not sure if the sensor needed to sit in my water or what but since then it has worked perfectly. I turned the alarm off because it screams when the water gets too high...every time you go into feed mode because the sump level rises when the return pump is turned off...annoying.
    One thing I REALLY DISLIKE is the pump is supposed to turn off if the container runs out of water. This is NOT THE CASE. I got up real early one morning to hear a whinning from my tank. Turns out the ATO pump was running dry and not shutting off. I checked the dial and it is in the middle so it should have shut off after a short time....
    Just a piece of advice, do not make the hose from the pump to your sump too long into the sump. My container and sump are the same height and it will create a suction and pull the water out of the container until it levels out. Did this the first time and it pulled 3 gallons out. :)
    If I had it to do over again I would not buy this since I have an apex now.
  • My Go To Auto Top Off By Alex on 3/13/2017

    I've had several different auto top offs from various manufacturers on my tank and this one is by far the most consistent and reliable. Not only is the set up easy, it just works. This is a plug, play and forget as it doesn't require any extra thought. It provides warnings, has back up redundancy built in and never overfills. Buy this and don't look back.
  • Tunze 3155 By Kazbo on 2/28/2017

    I have this product on my 265 gallon freshwater tank and works PERFECT !!!!
  • Definitely Worth the Money By Logan on 2/27/2017

    I have been running this ATO for over a year now and still would recommend this product. The optical eye sensor is the main feature that sold me on forking out the money for this ATO and it has not disappointed. I'm using a plastic 5gal container to hold water and using the ATO to pump that to my tank. The setup was simple and I have had no problems with the functionality of the ATO system. My only complaint is that the pump is pretty noisy for our situation. Our tank is in our bedroom and the ATO container is outside of my tank so when it kicks on in the middle of the night it can get a little noisy but not enough to wake me up. This significantly reduced my daily maintenance on my tank and keeps my water params more stable.
  • Problems day 1 By Adam on 2/16/2017

    This thing is the first product I've purchased from BRS that I've regretted. It failed in the on position the first night. So unhappy with this purchase for the price I paid.
  • Work with no flaws By Reefer2016 on 1/27/2017

    Up and running on my 75 for a month with no problems. Should have gotten a Tunze 3155 Osmolater Universal Auto Top from the start. My reefer friends that are using other ATO all seem to be having issues.