Get a ph meter, some raw apple cider vinegar, and a dropper for like kids medicine. If using tap watef, add one drop and immediately test ph. My rainwater is usually neutral (7). For me i add a teaspoon of molasses and it come down to around 6.6. Perfect.
I use rainwater, comes from the sky at 5.6 ph- just right for a hydro grow, though since rainwater should be zero ppm, that ph is essentially an empty value and shouldn't make any difference to your plants. Most people use tap water or RO water.
Depending on what medium you are growing in, you may have to supplement with calmag when you use rainwater. (You may have to add it when using tap water as well but not necessarily).
if you are in the uk most tap water is good. check the ec of the water with an ec meter anything under 0.7ec will work without issue. You need an ec meter to measure the strength of feed. when you mix your nutes just add an airstone to keep the water oxygenated. no need to let the water sit 24 hours if using an airstone and too be honest i never do that anyway and ive grown many thousands of plants.