I used RO water at one time for mall of my tanks. At that time I didn't know any better. One thing I did learn was that products are made to add to pure water to make it high enough in minerals for fish. An example of such a product is called RO Right. It comes with a little table that tells how much to add for different kinds of fish to get enough minerals for the fish's health. Once I figured out what I have for water, I stopped using the RO right and started mixing my tap water with RO for the fish that need very low mineral content and I use straight tap water for other fish. Water is different everywhere so it comes down to knowing what you have and what your fish need. I always find it best to get fish that are OK in my tap water but sometimes I just can't resist the more sensitive fish.
