Any time there is way too much of something in the water, the first and best step is to do a big water change, at least 50%. If you want to you can do another big water change immediately after. That pair of water changes should bring down the concentration of anything in the water to less than a quarter of what it was.
Do you fertilize the tank? How do you do it? Is your tap water high in phosphates (highly unlikely in the USA)? Did you calibrate the phosphate test kit to find out how accurate it is?