My darkroom only uses tank water. Came about as a result of our water restrictions we had several years back.

The tank water goes through four filters. A 20Ám washable screen filter takes out all the big bits from the tank water and this water is also used for the toilet and washing machine. Then in the darkroom there is an under sink unit with a 1Ám wound fibre to get rid of the sediment, a mixed bed resin filter for heavy metals (there is a lot of lead on my roof) and finally 0.5Ám carbon block filter. I change the filters in the darkroom every 12 months.

It works for me. But I have never measured the pH nor have I bothered to see what might still be there.