@Ace and @Ch3mnerd thank you for sharing all this information. 
This is a great thread! Instead of posting a new topic, what I was going to ask is relevant enough to this thread.
I’ve read what’s been said here and if I understand correctly dosing an acid can neutralise the chloramines and buffer the pH down to fix my quite lime-laden tap water in Southeast England. Dosing the acid is an option.
However, as I’d like to use rainwater where possible and “top-up” the water butt (which I pump out of for my watering), it’s more straightforward and convenient for me to use an RO unit (plus I already have one) to top up the butt. In the driest summer months, this may mean more “filling up” the butt than “topping up”.
My query is specifically around RO vs the acid-treated tap water (pretty hard water in my area):
- RO will remove the chloramines.
- RO, like distillation, will remove the vast majority of dissolved solids including the lime and trace minerals.
- Rainwater strikes me as being a kind of large-scale distillation and so rainwater to fairly pure and deficient of trace minerals(?) I can’t imagine it would pick these up in any noticeable extent from the atmosphere on the way back down.
- Conversely, rainwater having run through miles of rock etc. can pick up the trace minerals (including lime, if in limestone areas). But short of using well well water for our trees the most viable option for many may be an occasional additive of Trace Elements for plants, or a fertiliser which has these micronutrients in addition to the macronutrients.
In short, I was wondering if there are any negative implications of using RO filtered water due to lack of trace minerals?
If 100% RO is simpler for me, would I need to remineralise it or mix with half chloramine treated tap water as @Nicknjh23 is to reintroduce the trace minerals present in tap water?
Would acid-treated tap water be significantly preferable to RO, or vise-versa?