Phosphate comes in several forms. Organic and inorganic. (We test for organic) Although they may use some of one or the the other as well as nitrate (I feel the are primarily driven by this and Ammonia), the need silicate to build thier skeletons. Thus, when they appear it’s generally due to silicates.
https://www.advancedaquarist.com/2003/1/aafeature1
https://www.jstor.org/stable/1538545?seq=1#page_scan_tab_contents
Thanks for the knowledge. Reading some of that stuff makes me feel smart. Then I realize I have no idea what most of it means. So, I just go back to watching cartoons and coloring.