_Maq_
Member
I think the primary concern is bicarbonates. Hard water usually comes with high bicarbonates. They are unwelcome in themselves plus they push pH too high. Diluting such water with RO water is a sound approach.However I feel that the hobby has fallen in love with RO water just as people drink bottled water when tap water is fine.
On the other hand, I admit I haven't detected any negative influence of sulfates & chlorides. As for corresponding content of magnesium and calcium - i.e. water hardness - I do believe some species prefer them in the lower range.