This ATO has saved me from carrying heavy 5 Gal buckets back and forth across the house, and the two different sensor technologies give me a sense of security when it come to functioning properly and not becoming being stuck in the on position and overdosing kalk!

I found this top off unit easy to set up and comes with everything you need(no extra pump or valve needed). I would highly suggest this product as the first choice. The only drawback I found is when installing the magnet sensor mount in the tank (instead of a sump) I found the trim along the top of the tank will stop the outer magnet from going as high up on the tank as I wanted but, the remedy is as easy as a clamp or tank silicone adhesive. I am using an old 5 gallon salt bucket for my top off water. I added a float valve and enough 1/4" line to reach the RO/DI system in the basement so I dont have to hall the top off water up 2 flights of stairs to fill the bucket. I installed a ball valve on this line and only turn it on when the bucket needs to be filled. Works great!

I had some problems with this unit right off the bat...

Firstly, the pump didn't seem to be pumping anything. After several minutes of running (as evidenced by the whining noise), still no water was coming out. The ATO reservoir sits right beside the sump, so it only has to pump a few feet, and with about 14 inches of head. Finally, I primed the line using the age-old sucking technique, and from that point on it seems to be working.

The next, and bigger, problem was that the float switch, which is supposed to be the safety backup, was not working at all. Regardless of whether I lifted it manually or submerged it, no alarms would sound and the ATO kept pumping away. I contacted Roger at Tunze on their support forum at ReefCentral, and he confirmed that the magnets inside the float switch bracket were installed (by the factory) upside down. They are supposed to be at the bottom of the bracket, but these ones were at the top. Becuase the float switch itself operates on magnetic principles, the mis-aligned magnetic field was interfering with the switch, causing it to fail. After a botched attempt at fixing it myself, Roger is sending me a replacement bracket. No hassles--just a simple replacement.

The Osmolator and I still have yet to build a trust relationship. I've decided to plug it into one of my Apex controller's switched outlets, with yet another float switch (monitored by Apex). This way, the Apex can also kill the power to the top off if it detects levels getting too high. Redundancy is key.

The only other gripes I have with it, as other people have mentioned, are that it automatically runs the pump for several seconds when initially powered up (why?) and that the pump itself has a high-pitched shrill whiney sound to it, so it can easily be heard from outside the stand when it's running.

In general, after much manual testing of the optical eye sensor, I would say that it works quite well, and I am pleased with it. The price is hard to swallow, but I rent my apartment and it has hardwood floors, so I'm willing to take all precautions necessary to prevent overflow disasters.

Tunze customer service (Roger) has been very responsive, so in that regard, I give them (him) an A++.

This is a very nice ATO that keeps the level of water right where you want it. It saves me about 15 min a day at least from manually having to refill my sump, that's a lot of time saved after a while. well worth the cost, quality unit and you can tell a whole lot of thought went into this unit.

Love this system but the only flaw is the pump does not last, after 3 months it starts to drips after pump is off and sometime its a non stop drip. I'm on my 3 pump now and the system is only 9 months old. But other than that its a great ATO sytem, I just keep a extra pump now.

I just ordered my second Osmolator as the control module of my first one just died after 3 years. I didn't consider buying any other ATO system as this thing worked perfectly until it went out. The Osmolator is so precise at maintaining water level which is essential for growing healthy coral.

I like to have my 90 gallon reef tank as full as possible so it took a little time to adjust the over flow shutoff switch to the correct level. Once the sensors were adjusted to the correct height it has worked flawlessly. I'm very pleased with the Tunze 3155 Osmolator. It's worth its weight in gold.

This is a good product that works but has some flaws:

1. Every time the unit loses power or is unplugged and plugged in again, it will start pumping water for 10-15 seconds even if you are at the correct level or above.
2. There is an alarm to signial too much water. It's loud and can not be disabled unless you take the unit apart and move a jumper.
3. The eye didn't work correctly out of the box. I had to wet it and rub it with a clean cloth vigorously to get it to function. Once cleaned it worked. I question it's long term reliability.

Kinda wish I went with the JBJ ATO and saved 100 bucks.

Great product, set it and forget it. This product will make your life a lot easier.

I purchased this product to go with a Kalk stirrer I recently purchased from BRS. I had watched the videos from the knowledgeable staff at BRS. I have to say this auto top off is the best thing I ever purchased for my system. It is extremely easy to set up and you don't have the worry of it over filling since it comes with multiple safety features.

I was hesitant to buy this, given the premium price, but it keeps my sump level incredibly steady. MUCH better than the upside-down water cooler jug gravity feed setup I had before. The only odd thing about it is that it runs the pump for about 5 seconds or so any time you pull the plug and plug it back in. Not an issue unless you're turning it on and off a lot. Though that could happen if you have it on a controller set to only turn it on for say 5 minutes every hour. Just something to be aware of.

The Tunze osmolator is not the cheapest ATO, but I think it is the best. Worth the price for piece of mind.

I cant say how many times I let my ro run into my tank to top it off and forgot about it.this is set it and forget it.I have the pump in a reservoir that has a float switch connected to my ro.Nothing better than having one less thing to do to your tank.Pump is loud though just so you know but the water level is extremely precise.

After having a little trouble getting the osmolator working I emailed Tunze and washed the sensor with soapy water. This thing is awesome. I'm glad I spent the money and bought it over other float switches.

Works flawlessly, set it and forget it.

This is the best investment I have made in reef keeping. Works perfectly in my setup and has been running strong for 3 years plus. At the time I was worried about the cost but now I would not hesitate if I needed another for a second setup. The new magnet mount looks like a nice upgrade too.
A quick dip in vinegar every 3 months or so is the only maintenance I have ever done on the unit. You need to keep you outlet line above the storage tank BTW to avoid a siphon.

I don't know how many times I have come home to my sump pump running dry! No longer! I love not having to constantly check the water level in my sump. Product has exceeded my expectations and has yet to fail on me (not expecting it to).

Very easy set up and great product. Oh and make sure u clean the eye first before setting up. :)
And also the price should be on the $150 price range.

Hands down the best investment i,ve made in this hobby.This thing is dead accurate.Set it and forget it.Very easy to install ,easy to clean and very dependable.If you think the pump is loud just suspend it in the reservoir and you can barely hear it when it kicks on.Would recommend great product!

uses solid state sensor instead of float unit. does have a float unit as backup in case of primary sensor failure. has kept my sump level dead steady. pump initially had some noise but that went away after a few cycles and now is quiet. i did have an issue with some bubbles that collect on the sensor cause the unit to think the water level was low. fixing the bubble problem ended this problem.
seems like a lot of $ for an autotop off, but well worth it in my opinion.

Peace of mind that leaving your system for a few days will not result in a major disaster. Found no problems in setup, and it seems to require little, if any, further tweaking. I like to keep my tank covered tightly to minimize evaporation. If you fail to do this you will be adding lots of makeup water to your topoff tank. The other advantage of a secure cover is that the jumpers such as warsses do not go "carpet surfing" as they are very prone to do.

Only top off I will use. The electric eye is a billion times more reliable than a mechanical float switch.

Really easy to set up. Make sure you clean the "eye". Makes it easier to adjust at first.

One of the best purchases I have made for my reef. Makes life so much easier verses manual top offs and it is very reliable. The "eye" is accurate and it even has a float switch as a safety that will shut off the unit and activate a rather loud buzzer. I use a five gallon bucket of RODI water as a reservoir and it works great. If the bucket runs dry the pump will auto shut off after a few minutes to keep from burning itself up and again sound the alarm. I used the magnetic brackets to mount the eye and the "shut off" float a few inches higher up separately in my sump. One of the brackets is setup to hold both the sensor and float if that would suite your setup. The magnets are quite strong so you can be confident the "eye" isn't going to slide anywhere and change your sump level. One caution...I had an issue with it over filling my sump and setting off the buzzer the first few days. Couldn't figure out what was going on at first and then it hit me...and I felt foolish. This may seem like common sense but mind where you place the top off tube. I had tucked it down into the water next to my return pump to keep splashing noise down. Well...next to a full 5 gallon bucket full of RO water...once the pump started the siphon even after the pump cut off gravity continued to empty the bucket into my sump. So I shook my head and placed the tube a few inches from the top of the sump tank...above the water level in the bucket...problem solved.