What type of rocks will not affect hardness

[Duzzy]

Hi there,

I was wondering what type/sort of rocks do not affect water hardness. What should I look for?

To test them can I use hydrochloric acid (spirits of salts we call it here) or at least my dad does

Regards darren

[Red]

Slate......

[Lady Hobbs]

I just went out and collected a bunch of rocks from construction sites. If they were a weird looking rock, I poured vinegar on it to make sure there was no fizz. If it fizzes, do not use it.