The TMG label on the bottle (I am thinking 500ml bottle) peals off and you will find behind it a list of multi-lingual instructions.
The advice on dosing soft water is to use half the amount prescribed.
The advice on newly dosing TMG is to use half the amount prescribed for several weeks in order to check the general effect.
5ml is prescribed for a 50 litre aquarium. Scale that up accordingly.
I am dosing 3ml once a week to 30 US gallons. The effect is good in that leaf tone in general is excellent. I may increase the dosage slightly or at least review it after the hot weather has gone for good.
Flourish Trace seems like mineral water compared (borrowing from Rex Grigg's comments in another forum or discussion).
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.