02-09-2012, 07:16 AM
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Yes, it would slightly lower your pH as pure water has a pH of 7.
However, it also has a hardness (GH) of 0, and an Alkalinity (KH) of 0. That means you have no buffering capacity, and your pH can drop like a rock as the water absorbs CO2.
Typically people use RO if they have extremely hard water and want to soften it, they will mix their tap water with the RO water, but they won't use just RO water.
If you are just looking to slightly lower your pH, add some Malaysian driftwood to your tank, it will naturally release tannins in the water which will slightly lower your pH. Some people also use peat moss in their filter for the same effect.
Is there any particular reason you are looking to lower your pH? 7.4-7.6 isn't that bad, several fish will do okay in that. 6-7.6 offers a wide range of choices, wild caught fish are more picky than tank bred and may require the slightly acidic 6-7 range. There are some, however, who like basic water (live bearers come to mind). The fish profiles here can give you specific conditions needed, link is on the blue bar up on the top of the page.