Let me add a few things to Ben's advice:
1) If you have to use strips, the best are by Hach and LaMotte, the ones that measure about 6 things: TC (total Chlorine), FC (Free Chlorine), pH, CH (Calcium Hardness), T/A (total alkalinity), and CYA/Stabilizer. The TC is useless but the FC works fairly well and goes to 10ppm. The pH is tough to read and a cheapo OTO kit works better for pH. CH, T/A and CYA are very tough to read so don't rely on them too much, but they will give you a fair idea. FC is the best and easiest to read, especially if it's deep purple (Means your FC is 10 or above).

2) With the DE I'd like Ben to look again...it's a cartridge filter and I'm under the impression that DE works in DE and sand filters, but not carts. Ben will correct me if I'm wrong. (Several of us use a little DE in our sand filters to improve filtration, not a lot, but between 1/3 and 1 cup, depending on the water pressure).