While Doggie's comments are discouraging they are bang-on correct.

You have a problem and you are trying to get by using minimal testing tools, then relying on the pool store to find any problems. What THEY do is figure out what they can sell you. We always recommend that every pool owner get a good FAS-DPD based test kit (this means the chlorine test is FAS-DPD--the best. It measure Free Chlorine up to 50ppm, Combined Chloramines to 50ppm, and is the most accurate). They run from $50 to $80 depending on where you buy them (on-line is really the only option) and are worth every nickel. They are not an extravagence, they are critical to caring for your pool.

Other than that, Aylad's comments are what you should follow.

You MAY want to invest in a copper test kit--they are about $20 ro $30 from Leslies, but they will ENSURE whether you have excess copper in your water. At that point, we'll ask our metals specialist, Marie, to help you.