Since I got my tank in the summer of 2002, I'd been maintaining my water level using a 2 gallon jug set on top a PVC shelf I built in my sump. Every time the water level got below a certain point, GLUG, a bit of water would flow from the jug, and everything stayed pretty well stabilized. But while my solution was simple and reliable (as long as I remembered to fill the jug every two days or so), its flaw was that the jug took up a lot of space in my sump, and 2 gallons really doesn't last that long... A weekend trip would empty the jug. So a better solution was in order.
I started looking at various improved solutions, from hooking my RO/DI directly to the tank with a float switch, to expensive peristaltic dosing pumps, to simply getting a bigger jug. Then I stumbled across the Tunze Osmolator Universal at MarineDepot. It took me a little bit of guesswork to figure out exactly what "this sensor is insensitive to soiling, light or magnetic fields" and "even small waves or level variations cannot irritate the unit." really meant (Although the translator's English is better than my German), and there didn't seem to be any reviews out there on the web talking about this unit. Hopefully my description will be helpful to anyone considering upgrading their water top-off solution.
The Tunze Osmolator Universal 3155 comes with 4 main parts:
Tunze 5017 Water Level Controller: The basic brains of the controller system. It is hard-wired to the optical and float-switch sensors, with a shortish wire with a strange electrical connector at the end (more on that further down) for the pump. The power supply plugs into the bottom of the controller. Four lights tell you what's going on: A green LED is usually shown, when the system is balanced. If you fiddle with the optical sensor, you'll see this light blink on and off as it detects happy/sad water status. If the optical sensor stays off for long enough (about 2 seconds), the pump will switch on for a minimum of 10 seconds, along with a yellow LED indicating active pumping. 
Optical (IR) Waterlevel Sensor: This is a pretty fancy little gadget. As you can see from the picture, it has a slightly pointed tip that points down into the water. What you can't see from the image is that the tip is clear, with an LED package on either side. Actually, I'm guessing that one is an IR LED, the other is an IR detector. My uneducated guess is that when water contacts the bottom of this, it essentially causes the light being emitted by the LED to be channelled to the sensor, much the same way as fiber optics channel light by bouncing it off the interface between materials of different refractive indices. Anyways, that doesn't matter. Whenever water touches the tip, the Osmolator is happy. When it doesn't, the Osmolator is sad. Since there are no moving parts, this sensor should be very reliable. I'll have to see if over time any sort of salt crust develops. If it does, I'm sure just rubbing the sensor would fix any mischief. 
Magnetic Float Switch: This switch is your panic/cutoff. If for some reason the optical sensor doesn't register that the water has been replenished, the float switch will both cut the pump and sound an alarm. A constant beep tone. It's annoying, but it wouldn't wake me up, it's not very loud. But if I were awake, I'd likely hear it and try to do something about it. This is really the last safety, because if the system fills to the point that this alarm triggers, you've probably got a MASSIVE failure and your tank is siphoning into your sump or something equally evil, and if that is the case, it's probably power-failure-related, and if that's the case, this alarm won't be working because the Osmolator doesn't have a battery backup, and it won't have been the pump's fault anyways, because there's no power! But it's a nice thought. 
Water pump: The final piece of the puzzle. Just a normal pump. Has a power lead coming off it that hooks to the strange connector from the controller. When I say 'strange,' I mean 'poorly designed.' Instead of having a standard 'plug,' the end of the pump wire just has two stripped wires. You have to then screw these wires into terminals on the wire coming from the controller. It makes me uneasy to have bare wire near salt water. I may replace this in the future with some sort of real plug, or just solder the wires and seal them with heat-shrink. Low voltage or not, shorting the wire would NOT be a good thing. Also, I'd like it if the pump was either A) weighted at the bottom or B) had suction-cup feet. As you can see from the picture, the pump is tall and skinny. And top-heavy. - Power Supply: Really, a minor part of the system, but strange enough that I thought I should comment. The power supply is HUGE. For some reason, they include a unit that can be configured to supply anywhere from 1.5V to 12V (The Osmolator requires 9V). Why they would ship a supply that is four times larger than a simple 9V power supply is beyond me. Just make sure you have room on your power strip for this baby, for it will take at least 2 spots.
Now, the big question I had that I couldn't find answered on the web was: Does the pump have siphon protection? My sump tank is floor-level, and I wanted to place the refill tank around waist-level so I could refill it easier. I want to make sure that when the pump shut off, the water wouldn't siphon down into the sump and overflow the system. Well the answer (which seems obvious to me now, but didn't then), is NO, the pump is a regular old pump and has no protection built in at all. The refill container water level MUST be below the waterlevel of the tank it is filling. So I can't run the line from the osmo-pump into the sump. But the solution is simple. You just run the line up to the main aquarium, and place it above the waterline (so the main tank won't siphon back into the storage container). I have mine pumping into one of the overflow boxes, so the fresh/salt mixing still doesn't take place in the main tank, but there is 0% chance of siphon.
I don't plan on testing this, but there is also an option for the Osmolator that allows Kalkwasser dosing (replacing your evaporated water with limewater) that apparently keeps the solution stirred in the storage tank. It sounds like a pretty neat feature, but I'm instead planning on adding a calcium reactor to my tank in the near future so I'll never know for sure. If anyone wants to comment, please shoot me an email.
The Good:
- Set and forget... Even beeps if you forget to fill it Multiple safeties to prevent overflow
- Takes up very little space in sump
- Complete (but strangely worded) manual
- Accessories available to allow Kalkwasser dosing
- Less expensive that hiring a butler to do it
- More expensive than buying a 5-gallon water cooler jug and some tubing
- Sensors are hard-wired, making it difficult to run wires through holes
- Pump wiring has strange plug that seems to invite corrosion or electrical shorts
- Pump is somewhat top-heavy
- HUGE generic power supply
