I have very hard water where I live. Sometimes, the EC of my tap water can reach up to 0,7 mS and a pH of 8,5, which makes it almost unfeasible to use it for my hydroponic stuff.

Generally, using tap water for hydro is generally not recommended, because most people don't know what's in their water, and some things can be detrimental to the plants' health.

I do know what's in there tho. Here in Germany, we have extremely high quality water and every supplier has to make the lab reports public.

I sometimes "cut" my reverse osmosis or rain water with a few parts, usually 1/4 to 1/2, tap water. This gives me a bit of a pH buffer and reduces the water consumption of my RO.

But still, adding tap water just adds "stuff", and this stuff can't be used by the plants. It's still just "waste".

A lot of what makes the water more alkaline and "hard" are carbonates, especially magnesium and calcium carbonate. The cool thing is, that those are not very stable and can be removed easily.
If you boil water, they get converted into insoluble limescale, which settles on the ground. If you then just use the already boiled water, it's already less hard and more stable.

You can even do it more effectively by using food grade phosphoric acid, which I use as pH down at 10% dilution.

Experiment

I added a few drops (~5) into my water kettle and got it to a quick boil.

Here's the pH and EC of my tap water before adding or doing anything:

Here's how it looked like just after adding the acid and boiling it:

And here's the morning after:

The EC effectively got reduced by almost half!

What happened is that the phosphoric acid neutralized the Ca- and Mg carbonates and formed according phosphates, which are all insoluble and precipitate out of solution, making it cloudy.

Hereby adding something removes things. Cool, right?

What's even cooler is that other minerals, which didn't precipitate out of solution, can then act as buffer compounds, making the nutrient solution more stable against pH fluctuations.

Application in everyday use

The effect isn't super strong. Using RO or rain water is still not optional for me, I need it.

But this technique allows me to use way more "cheap" tap water than before, and adds some nice benefits too.

It's winter right now. Go and make yourself a hotty. You'll feel better for a few hours, and when it's gone cold, you can use it for your plants.

Don't just waste energy by boiling water. Make use of it and then make your plants happy with it! 😉