Question:
Rec.gardens and rec.arts.bonsai are getting it because of the use of dirst aquarium water as a fertilizer which people may not know about, the aquaria groups are getting it for the same reason?

Answer: I have plenty of experience using reverse osmosis water in Los Angeles!

When I lived there (3 years ago, for 10 years) I used to keep and breed killifish, a rather specialized (and very pretty) group of tropical fish. Some of them are rather demanding, and many killi hobbyists have home Reverse Osmosis setups.

A reverse osmosis filter works by passing water, under pressure, through a membrane. Somehow this removes a large number of things in the water, like calcium, magnesium, lead, PCB's etc. It does not remove chlorine/chloramine, although many people who don't know any better will tell you it will.

How much ``stuff'' is removed from the water by an RO filter is a function of how tigtly the membrane is wrapped around the core, the tighter it's wrapped, the purer the water, but the less water you get. For every gallon of RO water, it dumps about 9 gallons of ``liquid rock (LA Tap)'' down the waste water return. If the membrane is wrapped less tightly around the core you get a higher rate of water but it's not as pure.