Then, our advice is to purchase a good test kit (Taylor K-2006 or 2006C, see the testkit link in my signature) and not use the test strips. Find out what your CYA level is via the disappearing black dot test. The kit I recommended can do this test and most pool stores can do this for you but please do not let them talk you into buying anything else to add to the pool. Keep your chlorine at the appropriate range based on your CYA reading. (See the Best Guess Chlorine Chart in my signature below.)

Also, try the suggestions Janet gave you above regarding your stains.