I know the first thing I purchased when I first started in this saltwater hobby was a good RO/DI machine. We're on a acreage and I was paranoid about using the well water
The TDS measure about 660!
Anyways, after awhile I got a bit lazy (when I got my 4th tank) and started to just use the well water for my top up water, after all my betta's and goldfish do fine in it
anyways, I quit using the RO/DI machine over a year ago and things have never looked better! My corals are not only living, but growing! Go figure??
chicki