Whats the source of the waters hardness, calcium? I always found having to strip everything from water to eliminate trace amounts of nasties then spend lots to re add calcium and whatnot one of reefings great paradoxes
Except that in Seattle especially the water has such a high TDS of other metals and minerals that you'd be wise to strip them out prior to using it as you don't know what is in there, especially iron and possibily copper. Most of it is harmless IMO but is fuel for algae. In some parts of Seattle the water is almost hearty enough to be a meal.
Or a total reverse is the Alderwood Water District's water, it has a very low TDS, hardly any alkalinity but a pH of 8.4-9. The only thing left in the water is a bit of nitrate, phosphate, and added chlorine/flouride. If it wasn't for the additives and phosphate I think it would actually be suitable water. I think it
is suitable for a fish-only system. I've had no algae problems or mortality issues using this water for FO for years. But in a heavily stocked tank it contributes to the algae.
IMO the only reason to use stripped down water as your top-off is to avoid excessive algae growth.