Good morning. I have two freshwater tanks and I have no problems with algae except for some black algae on some of my plants in my older tank. I read it most likely is from too much light and/or high phosphates. I ordered API Phosphate Test Kit. The tank water came out about approx 4 ppm (it's a dark in-between color). My well water that I use for the tanks tested 4 ppm. There's my problem...so I think. I purchased PhosGuard and added it to my filter while pondering buying an under sink RODI system. A week later it's still testing 4 ppm. I've changed out the PhosGuard 3 times in two weeks at this point. I try diluting tank water with filtered water from the store... 4ppm. I test just the filtered Walmart water...4 ppm. I purchase 3 different bottled (purified) water from the store...4 ppm. Can it even be possible that all these water sources have extremely high phosphates? I tried to return the test kit and request a replacement, but Amazon won't allow you to return those types of things apparently. Should I try ordering Salifert or Fluvel phosphate test kit? Red Sea seems out of my price range. What do you recommend?