I've been using PhosFree (Leslie Pools) for years. I get large cheap 10 micron bags here
https://www.dudadiesel.com/search.ph...g&i=filterbags
I drip daily by mixing 10ml in with my carbon dosing
Clean the socks with sodium percarbonate in the washing machine, totally safe
Controls phosphate cheaply, easily and better then GFO in a heavily fed system IMO