Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The difference between RO and Distilled is not important as far as aquarium keeping goes, unless there is a problem with the production. Such as suggested above: Copper tubing in the process, or microorganisms getting through, or not enough maintenance to the RO membrane.
A simple TDS meter will tell you if there is something wrong as far as salts or minerals in the water. With good RO or distilled water the TDS meter ought to read 0, it might read in the single digits and that is still good.