On a tangent, the most ridiculously OTT restaurant I've ever been to proudly announced that "All our vegetables are cooked in water treated by reverse osmosis". This was in Los Angeles - perhaps not surprisingly. (Have to say, though - the vegetables did taste great!)
It's usually only used (with relation to aquarium fish) for very hard to keep fish which require a very specific water type...usually marines or malawi cichlids....the one big disadvantage being it robs the water of all mineral content and this has to be replaced.
Okay, further to my answer on the osmosis post, reverse osmosis can be done by applying pressure to the highly concentrated solution within the membrane. This has the effect of reversing the pressure differential, forcing the water from the high concentrate to the low concentrate. It is a very slow process, and requires quite high pressure, but produces the purest water.