Well, given the choice between dying of drought or drinking a bit of water with lime in it, I'm pretty sure your tree would choose the limey water;-))

I watered my own Acers in pots with tap water all the time for years, never bothered with the rainwater thing, and they were absolutely fine, and we have hard water here too, so really, don't worry about it.

We have the same lack of rain here in Hertfordshire and I've been watering my acers with tap water, I've done it in previous years when the water butts are empty and the acers are fine.

I had to empty my waterbutt yesterday so that I could pave underneath it - managed to move it when it was down to about 25 litres.
I have three little acers, they don't seem to mind Leicestershire tap water.
You could always add a bit of distilled vinegar - a cup of vinegar to a watering can of water should help.

Lime haters will also relish cold tea as it is slightly acid.

they wouldn't give it to humans if it had too much dangerous things in it really . your supposed to collect rain water or buy special water for monthly water changes in a fish tank but for over 30 years every month I take a third out and put tap water directly in its place . not a problem .

Thanks for all the info but we had one hell of a storm last night with torrential rain so the Water Butts are now over flowing, good job I checked that none of the taps on them were on the open position as I've left them open by mistake a few times LOL.
Nosey. I have heard that our tap water can make the fish feel itchy due to the lime in our tap water? When the butts are empty I fill one up and leave it a week & use that for the pond.
Have a Nice Day x

well if you can that's better le but my ponds 2000 gallons but it doesn't leek and I hardly have to touch it . my 200 gallon fish tank how ever I cant put rain water in and have been doing a third tank water changes for at least 30 years every month without fail . my gourami is 2 feet long and I have an 18" catfish and many others and there all fine .

Funnily enough, I was thinking of your rain water butts overnight - we too had a 2 hour lightning/thunder storm, with about half an hour's torrential rain between about midnight up to 2 am.

And my waterbutt wasn't connected last night ? while I'm waiting for a new concrete base to go off.