Basicly it's a membrane that allows water to pass through a special membrane that will remove about 95% of all minerals from the water. This water is too pure however to use with fish "as is" Therefore it is nessasary to mix it with a quantity of "normal" water as fish do need a certain amount of minerals and of course Bi-carbonates.
A normal off the shelf system will include a five micron prefilter and a carbon filter to remove any chlorine (that would destroy the membrane). You will often see all sorts of extras for "Polishing" the water but you don't need these. The pre-filters must be changed on a regular basis and this depends on the amount of water that has flowed through them (not the amount of time).
A standard unit will produce about 100 us gallons per day on normal tap pressure (60 psi) with 30% of that going to waste as rinse water. The rinse water can be used for other things like the garden etc. The amount can be increased by about 50% by adding a 12 volt pressure pump.
In most cases you let it work on a 24-7 basis by trickleing the water into the pond and letting it overflow to waste.
There are a few "Ands & Buts" that take a bit of explaining. There is a good thread on Koi Unleashed about this if you want to read more.
http://11.freebb.com/viewtopic.php?t...oiunleashedcou