Jet wash worked OK for me, followed by boil for couple of hours. IMO tho you have NO3 and PO4, the tests only go to like 30 ppb, but thats still 10 times higher than in the wild reefs. I like Glenns use of iron chelate, breaks down slowly over the day, removes phosphate continually and since its already in the water, iron molecules are ready to grab phosphate a soon as they become available, better competition for algae than GFO reactor. Sand beds are my personal first line of suspicion though, if you have gram levels of PO4 in your sand from years of heavy feeding, its going to take a long time to get rid of it. Remove sand completely IMO, then leach the PO4 from the rock for a few months with chelated liquid iron. Algae need nutes. Also, your organic phosphates might be higher than you realise, testing for inorganic PO4.