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
  • Super easy install of the Tunze Osmolator 3155 ATO & why we love it.
  • Digging deep into the Tunze Osmolator 3155 Auto Top Off - BRStv

Osmolator Universal 3155 Auto Top Off - Tunze

96% Recommend this to a friend (321 of 336)

SKU: 202307

reg. $199.99

Special Price: $139.99

In Stock: 145
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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


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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  • The best long term ATO By Eddie on 10/25/2017

    I have owned a couple of Tunze and a Neptune. Neptune is little more finicky with the water level and the very hardly audible alarm. The Tunze Has been a better product long term. The Tunze does not have that huge plate and float ball of the Neptune. Tunze very heavy duty with very strong magnets. Always make sure your output is above the level of the reservoir or place the hose in a camouflage PVC.
  • Great buy! By Carlos on 10/24/2017

    Was a little hesitant to purchase this item, its on the higher end $ wise for a top off. But this unit brings everything you need! and has three fail safe features which for me was the key since my top off tank is 60Gal. I read multiple reviews both here and on forums and this was the highest recommended ATO.
    The only feature I wish that this unit doesn't have is suction cups to hold the pump in place. I solved the issue by zip tying it to a pice of egg crate which kept it in the correct position. Not a big deal as many people use this with Kalk and it wouldn't be ideal to keep it on the bottom.
  • great By PETRO on 10/5/2017

    going to by one more for new tanc
  • It just works By Ndrwater on 10/2/2017

    Best ATO out there. Hands down. It just works. Never have to worry about a thing.
    Have one for each if my systems and wouldn't do one without the Tunze 3155
  • Just awesome ! By Jerry on 9/26/2017

    What can I say about Osmolator Universal 3155 Auto Top Off - Tunze. Easy to install took me a matter of minutes, being my first one I researched long and hard and this one seemed the best. So far it has been great. I even overfilled my tank myself by mistake and the alarm went off . So far so good a good buy for sure.
  • One of my best purchases yet! By "It's so easy!", they said.. on 9/20/2017

    Wondering why I didn't install one sooner! This thing is a huge time saver. If you're on the fence, just get it. You will not regret the purchase. Mine has worked flawlessly for months. The only trick to know is to flip the pump upside down & release trapped air bubbles if you ever allow it to run dry & it won't pump after refilling the container.
  • Sweet System By Gary on 9/10/2017

    So I watched the video of Ryan hooking up the Tunze ATO. And it literally took me 10 minutes, taking my time, to set it up. The alarm noise is a hoot. Doubt anyone could sleep through that. Really liking it so far. Teamed it up with a Crystal ATO reservoir from Trigger. Really sweet. The wife approves. Win-win!
  • SOLID PRODUCT! By Jonathan on 9/7/2017

    I thought, "Meh, I won't mind refilling my freshwater by hand daily"........ well that lasted for two weeks. Absolutely love this thing! Took some tinkering to get it right, but after a day or two I have it dialed in perfectly! Keeps salinity on point and now I can easily dose Kalkwasser. One day I noticed the monitor said "too low," but my ATO bucket was full so I turned it off and back on and everything is peachy again :) You won't regret this product!
  • Quality By Matthew on 8/27/2017

    Have had for a couple months and has worked perfectly. Tested the safety shutoff and works great. Don't have to worry about sump overflow anymore.
  • Great By navathehutt on 8/27/2017

    This german made product has lot of magnets and clips and accessories to fit your need. Extremely simple to install. You need to prime the pump when it first starts. Move it around so it can suction the water. I had some cheaper alternatives in the past but this thing works exactly. Didn't have any siphoning after shutoff either.
  • Good just one issue By Kenae on 8/23/2017

    This is a great ATO I've been enjoying it the whole time. The only thing I don't like is the clip that holds the water line into the tank/sump I think that is pretty poor but all in all it's great
  • Easy to install and use By Felix on 8/12/2017

    The product is easy to instal. Has good instructions and works as advertised. I paired it with the reefkeeper lite.


Ask a question

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
Is the retaining magnet waterproof?
Question by: Joe b on Nov 18, 2017 1:39:00 PM
Yes, the inner and outer magnets are completely sealed and can be submerged.
Answer by: Charlie (Admin) on Nov 18, 2017 6:44:00 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 is the power consumption of this unit?
Question by: Patrick on May 12, 2017 2:37:00 PM
Hey there,
The power supply has 12V, 1600mA maximum output, connected to a watt meter with the pump running, the draw at the outlet is generally around 8W at 115V with the pump running. The entire system uses 1-1.5A at 12V depending on pump speed setting and head pressure.

Happy reefing!
Answer by: Zack Wilson (Admin) on May 15, 2017 10:45:00 AM
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
Can i purchase this item with a australia (SAA3) wall pug not a north america one thats shown in the photo?
Question by: Matt Drew on May 24, 2017 2:00:00 AM
Hey there,
We appreciate your interest, but I'm afraid our distribution area is limited to North America and all of our equipment is wired for standard US 110v electrical.

Have a great day!
Answer by: Zack (Admin) on May 30, 2017 3:57:00 PM
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