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

97% Recommend this to a friend (279 of 287)

SKU: 202307

reg. $209.99

Special Price: $188.99

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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
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
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
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.  



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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  • Great ato. Works as advertised. Pump fits in 5 gal jug By Chris on 5/17/2016

    This top off works really well. The only thing to remember is to prime the pump but after that it runs like a champ.
  • So long to my KW drip By Eric on 5/6/2016

    After months of setting up an evening kalkwasser drip (and often forgetting to set it up), this is a real time saver. Once a week now I fill my top-off bucket and it keeps my sump consistent and slowly doses my KW. I'm a fan.
  • You will HAVE to replace in 2 years By FlyinBryan on 4/26/2016

    My recommend to buy is very tepid. You will only get about 2 years max out of one of these units. I really want to like it, but, the eye stops functioning after 2 years. All the cleaning of the eye will NOT make a difference. I have 1 hour to get to bed and get up in the morning for an early flight and I'm in travel he!! week, 6 flights. I can NO longer rely on this essential piece of equipment to maintain my tank. I will probably loose several SPS as a result of my jacked up tank all because this ATO failed --- AGAIN. This is the 3rd one I've bought that does this, it is NOT an isolated one off case at all. Poor quality in the long run. I will never buy again.
  • best in the market By eric on 4/10/2016

    I feel safe that this will not fail and works great with the Apex or without
  • No worries By MARIO on 4/5/2016

    I purchased the second one, and as for piece on mind extra pumps, one pump failure thus far. It is a good product, price but good..
  • Nice product By Knuckledragger on 3/16/2016

    The instructions and a bit weak but I was able to look online and get the info I needed. Overall this is a bit expensive for what it is but works great. The pump is louder than I expected even though I have it turned down as far as it will go (nano setting).
  • Best out there By Mike on 3/14/2016

    After my 1st two ato's failed (smart ato and jbj) I finally bought the tunze. It is so much better it's not even close. Wish I had bought this 1st and saved myself the $$$ and headaches.
  • Works but its very sensitive to the water level. By David on 3/3/2016

    I watched the BRS youtube videos and listened to Ryan talk about how he prefers this ATO to the others on the market. I bought into the hype and bought it. I am running a neptue apex on my system and I realized after I bought this ATO, that with the Salinity probe I'm running i could have built a DIY version that would have done a better job of keeping the salinity at a set value. The osmolater is great for filling from the tank with Rodi water. But my Salinity shifts from a high 36 to a low 35 ppt rather frequently. Its best used on a system that isn't running a controler like an apex or reefkeeper. With a controler you can buld a much more consistent ATO from DIY parts.
  • Best ATO By Jeffrey on 2/20/2016

    This is a no brainier. Not worth the risk to buy a cheaper product.
  • Flawless By AJ on 2/18/2016

    I've been running this for over a year now and it's been flawless. No need to worry when I travel.
  • 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.

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