I use nothing but filtered rainwater in the darkroom. My 3 filters are 10µm, 0.5µ and then one which takes out the heavy metals, from memory it is carbon based and not fibre. Can never remember the details of what that one is. I use the heavy metal filter as there is a lot, I mean a lot, of lead on our roof.
I mix my developers from scratch and have not had any concerns at all.