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Re: DI and RO

 >After reading those messages on DI water, I got a simple question: what is
 >the difference between DI and RO water?

I'll take a shot at this one :-)

RO is Reverse Osmosis, and is the common "one stage" filter that people buy 
to filter their tap water for aquarium uses. It works by using a 
semi-porous membrane to filter impurities out of the water based on 
particulate size (keep in mind that the size is *very* small -- large 

RO cartridges are commonly preceded by an activated carbon filter and a 
regular particulate filter. The particulate filter is to remove the large 
particles like pieces of rust and other sediment that could clog the pores 
in the activated carbon or the RO membrane. The activated carbon is used to 
strip out the nasty stuff like chlorine that can damage the RO membrane.

DI is De-Ionization, which works based on chemical filtration. Certain ions 
in the water will have a + or - charge and will chemically bind with the 
-'ly and +'ly charged compounds in the DI cartridge. This filter cartridge 
works on the level of atoms.

The ultimate filters combine all of the above, with a mechanical (large 
particulates) filter, activated carbon, RO and DI, in that order. The 
resulting water approaches distilled water in purity. Such systems are 
excellent for providing very pure top-off water but will need to be 
conditioned before use in an aquarium.