All the time I have had plants around my house I have used tap water that has been allowed to sit for several days to let some of the chemicals evaporate out of it. This was for my tropicals along with any succulents I may of had . It seemed to do the trick but eventually always left that kind of whitish chalky look on top of the soil .
I had heard that using distilled water or rain water is much better for all plants. Is this true and which of those two is better? We have had a lot of rain water this summer and more to come so should I be collecting that ?
Thank you
Wendy

For cacti in general, I use rainwater, just collected in a water butt, and not filtered or treated in any way. My tapwater is quite hard, as we live in a limestone area.

I know that many Mammillarias come from places where the underlying rock/soil ha slots of limestone in it. So in theory it shouldn't matter. But using hard water also creates white deposits where the water evaporates, so can leave unsightly marks on the plants, to the extent that in some old plants there's a really white ring around the neck of the plant - which looks bad, but perhaps doesn't harm.

I use about 40 gallons a time when I water the greenhouse, which isn't a huge cost, maybe only a few pounds, but if I can use natural resources, then I will.

thanks Chris, I should of started collecting the rainwater here this past spring and into summer. We have had about 3 times the usual rain fall. Wonder if that hints at what kind of snow we will be getting too hmmmm
As I don't have rain water I have began using distilled water on all my plants , trops included and so far they are all seeming to like it.
Have a good week
Wendy

I'm sure that'd be fine. I boil my tapwater to use for my seedlings as this destroys most of the hardness - but doesn't half fur up the kettle!

Chris, I know what you mean. I love my cup of tea in the evening and boiling that hard water in the kettle. Makes for nice lime covered sides to a kettle. I think I have gone through about 5 kettles in the last 4 years .
Take care and again thanks
Wendy