It’s totally worth the $50 for a Hanna. Sometimes can even get them used for cheap.
I would first take a water sample to a lfs and have it tested to be sure. Or a friend with different test. Or just get a different test
If it is 10- maybe small doses of Phosphate E can get it down but go slowly! Like .5 or even .25 every few days.
Most likely leaching from rocks as dry rocks are pretty young. Maybe some beneficial bacteria in a bottle to give a little jump start.
Never had good results with API phosphate tests.
Good luck my friend!
