I asked the same question to my lfs yesterday. The guy that I deal with said that it's better to have RO WATER because it's all pure, nothing is added and it's best for your aquarium. He said that Distille water has some additives. I did a water change yesterday with RO and my fishes look happy. I used to use Distille, but since I asked my lfs, I'm sticking with RO from now on.