Overall it is a great product. I have not experienced any malfunctions and the holder for the float switch and eye is very convenient. It does what it is supposed to do. However, I do have a few complaints.

1) The pump automatically pumps about a cup of water each time the pump is turned off. That means, if you dose kalk with your ATO as I do and your APEX turn the pump off because the pH is too high, as soon as it turns back on it pumps more high pH into the tank. Not a good feature. I'm sure it's to maintain constant flow through the tubing after it has been resting, but a manual button for this would be better

2) When the ATO alarm goes off, the only way to turn it off is to power down the ATO. Which means when you turn it back on, AGAGIN, it pumps more water into your tank. A button for this would also be better.

3) Eh...I guess it could be a little more attractive. Kind of an ugly black box.

Finally got around to setting this up yesterday and it was easy as cake. 5 minutes out of the box and rolling. Everything you need for easy, dependability, and redundancy - could not have asked for a better ATO. Must have.

GOT THIS ATO A WHILE BACK, JUST WANTED TO TEST IT OUT BEFORE I POSTED A REVIEW..."IT'S A WINNER" LOVE THIS ATO UNIT. VERY ACCURATE - ALSO LOVE THE MAGNET DESIGN. I ALSO PURCHASE A SPARE PUMP, IN THIS HOBBY YOU NEVER KNOW WHATS GOING TO HAPPEN. AT LEAST WHEN IT DOES HAPPEN , I'LL HAVE A SPARE.

definitely worth getting easy to set up, keeps water level perfect, no problems

the tunze osmolater is a must have for any lazy reefer lol, it keeps the salinity at a set point where you want it i love it & i got it for a cheap price on black friday..

Does the job perfectly. my water level is always constant, alarm has never gone off.

Keeps the warter right where i want it. i just wish i would of got this one a long time ago

I was a little skeptical after the instructions stated that this pump was not meant to handle large tanks. Let’s just say the pump works fine for my 370 gal. I have the ATO dripping into the sump and don’t take long for the water level to stabilize. I love this product; it keeps my skimmer stable and saves me a numerous trips of dumping water in tank.

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.