RO/DI water bad for tank?

[hockey nut]

I have been using a new Ro/Di system for about a month now and noticed recently that most of my fish in three different tanks are showing signs of stress or are sick. My question is, do you need to add elements back to the Ro water? I wouldn't be to concerned if it was one tank but all three tanks are affected and after I did water changes on all three. Anyone else experience this with RO water?