I know RO water can strip your fish of minerals and trace elements due to the lack it the water and the RO process makes it effectively *thirsty* for them. Distilled water is also stripped of those elements and unless you are buffering the water before you put it in your tank to replace them to the approprate levels this may be causing you the issue. I had a major fish loss when i went to strait RO water on my 75 gallon now i do a mix of about 1-2 gallons tap per 5 gallons of water