If you've ever seen those little mini spouts on a kitchen sink, that's an RO
unit. They're made for purifying our drinking water. If you buy an Aquarium trade one, you'll pay much more... they're probably better, but as long as you get a 0 ppm
reading, thats all that really matters, it's done it's job. The ones on ebay are all designed for home consumption.
You can really spice up the argument for one if you have a fridge that's got a plumbable ice machine in it. hook your RO
up to that for some purified ice cubes.
Try and get the de-ionizer chamber on it if you decide to get one.