If you can use distilled water,i would,just that,Theres no ppm and it makes a difference when adjusting ph and adding nutes..it is what it is,,if you lower the ph to 5.0..its stayin at 5.0..It doesnt fluctuate byitself like tap water until you put it in the res.
I keep a mixing tank..a lil bigger than the bp...im usiong tapwater with a ppm of 500..i add 6 gallons in the tank and 5ml. of down per gallon,that brings it to 5.8 everytime,well i know its gonna rise a lil so i lower it to 5.0..wait a day or two and its at 5.8-6.0(PERFECT)..then i add my nutes and have no troubles