Osmolator Universal 3155 Auto Top Off

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Out of stock

Discontinued by Bulk Reef Supply
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NEW VERSION AVAILABLE: Tunze Osmolator 3 ATO System

NEW VERSION AVAILABLE: Tunze Osmolator 3 ATO System


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.


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



  • Max Head Height: 7.2 ft
  • Tubing Diameter: 7mm OD | 4mm ID
  • Tubing Length: 3 Meters

If you need a pump rated for higher head pressure, check out Tunze's Turbelle High Jet Pump designed to replace the stock osmolator pump



Click Here

More Information
UPC 4025167315510
Dimensions (Pump) 3.75"H x 2" Diameter, (Magnet) 2.5" Diameter, (Controller) 4"L x 2.5"W x 1"H
Max Tank Thickness 3/8 in.
Included Mounting Magnetic, Hang On/Bracket
Control Type Digital DC Control (Controller Included)
Pump Included Yes
Pump Model Tunze 5000.020 Metering Pump
Power Type DC
Warranty 2 Years
Water Level Detection Float Valve or Switch, Optical Sensor
Alarms Audible, Visual
Safety Features Overflow Timer, Run-Dry Protection, Empty Reservoir Alerts
Max Head Height 5 -10 Feet
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Product Questions

What is the gph rating on this pump?
Question by: Stetson barkes on Dec 11, 2023, 5:29 AM
Hey there,
Thank you for reaching out to us. 15 GPH. Please let me know if you need anything further.
Answer by: Thomas Wofle (BRS Staff) on Feb 1, 2024, 1:01 PM
Hey BRS team, I have one of these but need a new tube to feed the water from the pump back into the sump... what should I order?
Question by: Richard Stevens on Jul 30, 2023, 5:18 PM
Hey there,
Thank you for reaching out to us. You would want to order this.
Please let me know if you need anything further and take care!
Answer by: Matt Lucero (BRS Staff) on Aug 1, 2023, 8:18 AM
Will this fit in the the back of a 40 gallon long Innovative Marine all in one tank?
Question by: Sean on Mar 9, 2023, 5:42 AM
Yes, this would :)
Answer by: Ash (BRS Staff) on Apr 21, 2023, 2:00 PM
Would this work in my icecap 24 3 gallon reservoir
Question by: Devin on Nov 13, 2022, 12:13 AM
Thanks for the question! This will work on the IceCap 24 sump with reservoir. Please let us know if we can help with anything else!
Answer by: Kyle Thomas (BRS Staff) on Nov 22, 2022, 10:10 AM
Hi There !
I have a plastic drum 26 1/4 height , 14 " D . Do you know the pump is enough pressure to push water vertically ?
Question by: Margaret on Jun 22, 2022, 2:31 AM
The max head height this pump can do is 7.2 ft.
Answer by: Ashley Hoglund (BRS Staff) on Jun 23, 2022, 2:40 PM
Is there a option to purchase without the pump? Planning to hook it up directly to my water source
Question by: David R on Nov 15, 2021, 3:57 PM
Hello, the Universal 3155 ATO Controller is not available for purchase without the pump. Thanks
Answer by: Christian Jimenez (BRS Staff) on Nov 15, 2021, 5:48 PM
What i sthe dimension of the pump?
Question by: Rad on Sep 12, 2020, 10:59 PM
The pump is
3.75" Tall
2" wide
Answer by: Matt Lucero (BRS Staff) on Nov 5, 2021, 10:02 AM
Can you tell me who to contact to purchase a replacement plastic guard for the float switch for the osmolator
Question by: Matt on Apr 8, 2020, 9:03 PM
Hi Matt,

Your best bet is going to be contacting Tunze USA for any parts.
They can be reached at 512-833-7546 or tunze@sbcglobal.net.

Happy Reefing!
Answer by: Nick on Apr 13, 2020, 5:00 PM
I'm thinking to use this to top off with Kalkwasser, are there any issues with that? Also, my sump is a 150 gallon black plastic stock tank. Any issue with the magnet?
Question by: Nick Gialias on Feb 1, 2020, 10:51 AM
Hey there,

Kalkwasser will degrade the life of the pump and is not covered under warranty. The magnet is fully coated and can be submerged.

Take care
Answer by: Thomas Billington on Feb 6, 2020, 9:51 AM
I would like to use this in my sump. Does this package include everything that I need except for a water reservoir?
Question by: Laura on Jan 18, 2020, 11:33 AM
Hey there!

It does come with all necessary items except the reservoir!

Take care
Answer by: Thomas Billington on Jan 22, 2020, 2:45 PM
How far can the pump be from the controller? Or, how long is the wire from the pump to the controller? I need to have the reservoir in a different room from the tank. The rise is only 2' but I need to go through a wall and about 10' from reservoir to sump.
Question by: Brett Petersen on Dec 18, 2019, 7:12 PM
Hey there-

The pump has 1.2 meters of cable (~3.9'), the controller has 1 meter of cable (~3.2') that connects to the pump, overall you have roughly 7ft total. You can extend it with speaker wire if you have to, but you only have 9ft of hose with it. If you have any other questions let us know.

Happy reefing! c:
Answer by: Brad Ohmann on Dec 19, 2019, 2:56 PM
What are the dimensions of the pump at its widest points? I have an ATO reservoir with a small opening, and I'd like to know if this pump will fit through the cap.
Question by: Jerry on Apr 16, 2019, 10:26 PM
Hi there! The pump measures 1 7/8" at the widest. Have a great day :)
Answer by: Joe on Apr 17, 2019, 6:38 PM
Can this connect to the Apex controller?
Question by: Tim Law on Nov 23, 2018, 10:35 AM
You can plug it into an outlet for basic on/off control, however, there is really no control function outside of that with an Apex system. Most people plug the Tunze ATO systems into the Apex to turn it off during maintenance or feed modes, so they do not need to remember to turn it back on.
Answer by: Charlie on Nov 23, 2018, 2:25 PM
Can you tell me the diameter of the wet side unit? I need to make sure it will fit inside the return pump side of my bio cube 14. I’m worried the sensor will not fit due to the narrow area I have to work with.
Question by: Michael on Jan 18, 2018, 9:34 PM
The diameter of the magnet is 2.5".
Answer by: Charlie on Jan 19, 2018, 9:15 AM
Is the retaining magnet waterproof?
Question by: Joe b on Nov 18, 2017, 1:39 PM
Yes, the inner and outer magnets are completely sealed and can be submerged.
Answer by: Charlie on Nov 18, 2017, 6:44 PM
I have the -osmolator nano 3152 ATO and I was wondering the water level controller can be adapted to it
Question by: Juan Resendiz on Nov 9, 2017, 1:20 AM
The Tunze Osmoaltor 3152 is a stand-alone ATO controller and cannot accept the Tunze Water Level Controller.
Answer by: Charlie on Nov 25, 2017, 10:53 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 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 on May 30, 2017, 3:57 PM
What is the power consumption of this unit?
Question by: Patrick on May 12, 2017, 2:37 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 on May 15, 2017, 10:45 AM
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 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 on Jan 11, 2017, 11:30 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 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 on Jan 9, 2017, 9:18 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 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 on May 3, 2016, 5:01 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 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 on Apr 19, 2016, 4:05 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 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 on Apr 13, 2016, 6:29 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 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 on Apr 6, 2016, 11:32 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 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 on Mar 22, 2016, 2:08 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 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 on Mar 21, 2016, 6:50 PM
Will the sensor fit in the sump of an innovative marine 30L ?
Question by: Ian on Feb 9, 2016, 8:33 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 on Feb 9, 2016, 6:11 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 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 on Jul 7, 2015, 1:03 PM
Will this work with the Apex controller?
Question by: Jim on Aug 3, 2014, 11:31 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 AM
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 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 PM
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 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 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 PM
Hi David,
The max head pressure on the osmolator pump is 7ft.
Answer by: Bulk Reef Supply on May 10, 2014, 11:50 PM
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 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 PM
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 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 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 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 AM
How much head will the pump push
Question by: Jeff on Nov 28, 2013, 2:50 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 PM
what pump do you suggest to go with the3155 osmolator?
Question by: BILL on Nov 13, 2013, 7:37 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 AM