Hey dlove,
buying distilled water is not really necessary, tap water should be just fine. keep in mind that adding
nutrients to plain water will affect the pH so do all your adjusting after you have added your nutrients to your reservoir. also, your pH will fluctuate more depending on your res size(the smaller the res the the more pH movement you will see) and plant metabolism has a lot to do with pH, a little movement thru-out the day is fairly normal. try testing your pH at the same times of day. i will agree with the others that vinegar is not the best pH adjuster. liquid or powder which ever you prefer just not vinegar.
one other thing i will suggest for your unwanted algae growth is Hygrozyme. it is enzymes that help break down organic matter making nutrients more available to your plants and will help keep algae growth in your system to a minimum and is comparable with most
nutrient lines.