carey
Aquarium Advice Addict
It removes everything. Chlorine, heavy metals, nitrates, phosphates, everything bad for your tank is gone. 99.9% pure water. Everything your fish, inverts, and corals need is then replaced by your salt mix. I didn't use one when I started my tank as I wasn't familiar with its benefits. Trust me you want one, saves alot of headaches.
I also started my first sw tank with tap water and it wasnt pretty. lol It took months of water changes to undo the damage caused by the tap water. Phosphates can leech into the rock making it even harder to get out.
The ro/di water is just one of those things that can make keeping a SW tank alot easier and more pleasant. Just gets rid of everything that is and could be bad for your tank. Most fish stores sell it by the gallon, usually about 50 cents or so. I also was using a water machine outside of my supermarket that was 30 cents a gallon until I got my ro/di unit.