Just a quick 5c from me!
I didn't use RO water when I started up my system and had endless problems with algae and cyano outbreaks! And then sicknesses like white spot due to unhealthy water for the fish! Almost all the first fish I bought has died.
RO water does not contain any harmful materials like chlorine, bacteria, other impurities and total dissolved solids.
By starting and maintaining with RO water only you will have an extremely healthy system.
Let me try and put it like this: Some reef salt manufactures pump reef water to their facilities and then heats the waters to evaporate. All the salts, trace elements and minerals stays behind. Remember that solids in water cannot evaporate. Now, you the reefer buy the salt and in order to replicate the exact same water that is found when they first pumped it for processing, you need to add the cleanest from of water available. By adding tap water for example you are basically adding more solids like minerals, fluoride, calcium, chlorine, bacteria etc. This causes massive imbalances in the system and you can guess, the death of live stock and algae and cyano problems!
Hope this clarifies..