We have a natural finish and never had problems until we switched to a mesh cover 2 years ago. Now we have stains in the shallow end. We did the Vitamin C test with negative results. We were advised by a local pool supply company to use Sparkle; however, the directions say to backwash. We have a modular filter - not sand or mud, so no ability to backwash. The company that built our pool said that Sparkle would ruin our filter, yet several posts state that only vacuuming is necessary. What is the truth? Will Sparkle ruin my filter? If so, what else can we do to remove the stains short of draining the pool?