RO/DI water is not good for you to drink (or to put straight in a F/W tank). The principles behind this are simple....
- RO/DI = 0 ppt salts (Na+, Cl-, K+, Ca++, Mg++)
- Human body = lots o' ppt salts
- This means that salts will leave your body, in favor of the low concentration RO/DI water
I am not sure that this action can remove the molecules from your teeth, but it definitely can screw with your system. Water in bottles is only RO, and has some minerals left...but just using a brita or pur to remove the 'taste' would be the best thing. We need all these ions for our body to function properly (nerve impulse, muscle contractions)....
As for freshwater fish, it will definitely kill them in a short period of time if there is no addition of salts to the water. FW fish have a concentration of ~9ppt salts in their blood, and a low concentration of water.....put them if RO/DI.....and that gradient is even larger! The ions rush out and water rushes in through the gills and skin, resulting in a very unhappy, very dead fish.
If you want hard water just at some of our saltwater salt, ~3g/L (or 3ppt). This will add plenty of ions to help the fish osmoregulate. For African cichlids, you would also need to add some buffers to help simulate their natural water.
Hope that helps.