If your phosphate test is a 10, then your water quite likely registers much higher than a 10 since the common aquarium/pond phosphate test kits are only testing for orthophosphates, that is reactive, plant available, esters or salts of phosphates, rather than metaphosphates, that is condense, complex organic, only soluble after digested by an enzyme from microorganisms or plants that create the orthophosphates as the result of organic decomposition.
Generally, I found phosphate tests stir too much worry from folk, that is more than needed and causes folk to unnecessarily overreact. For water gardeners, registrable phosphate levels is common due to all of the plant, algae, and fish waste decay.
An effective natural phosphate reducer is anything with a high iron content. Iron has a very high attraction to phosphate, that is much like how calcium has a very high attraction to carbonates (CO32-) in higher water pHs. Iron is one of the major nutrients that is most often lacking in pond waters due to its high demand for chlorophyll production and photosynthesis. Some folk's water is naturally high in iron so, even if the phosphate test it high, it is no worry since the iron content counteracts it.
Water hyacinth is a good "canary" to tell you if you have an iron deficiency in your water. Oregano, basil, and petunias are also good iron "canaries". If these plants have any slight yellowing, then it is most likely due to low iron content, but, since other nutrients also impact the production of chlorophyll, which is what creates the "green-ness", then the yellowing could be due to particular string algae excreting a poison forcing the plant to have a deficiency or some other nutrient deficiency. The plant will still grow and "try" to function so to survive, but it will never truly thrive, and, when it is being retarded due to any deficiency, then this means it is also consuming fewer nutrients, including phosphates, out of the water.
Iron is not a toxic metal to fish, but, if you have too much of a good thing, then eventually it will hurt the fish and this threshold can be met quite fast since the iron products are quite concentrated. The thread,
Water pH and Peat Moss, should hopefully give you some insight on how to properly test and diagnose iron. You can also mix in a
plant medium product, called Laterite, which is high in iron, to your aquatic plant pots, and this will help the plants consume more phosphates due to Laterite's high iron ability to attract the phosphate.
