Based on the assumption that more Fe causes thread algae. Something I've never been able to show.
Perhaps more an issue of "waste", it ain't cheap.
Softer water seems to maintain the chelation longer I would guess.
Less nutrients in general are present in softer water(Less carbon, Ca, Mg, SO4, K+ etc).
But plant growth is similar in soft vs hard if not better in hard water as a generalization.
A couple of plants seem to do better in soft water but 2-3 out of 300 seems like a trend to me, 99% of the time is good enough for ageneral statement.
Regards,
Tom Barr