As usual, Watermom's got you headed in the right direction...the only other thing I can add at this point is that when you take your water sample from the pool, get it by inverting your cup/vial/whatever you're using, submerge it in the pool approximately 12-18 inches, then turn it right side up and allow it to fill. Also, you don't want to take it from an area where you've just mixed anything new into the water. Always give it a chance to circulate, then go somewhere else in the pool (I get mine in front of my skimmer, which is almost in the center of my pool) and get your sample. If you take it from where you just added tap water, then all you're testing is your tap water.

Janet