Do you seen any precipitation? Lanthanum is easier to use that GFO, ime. The liquid you have must be very dilute. I would double or triple the dose and remeasure shortly afterwards to see if you can detect lower levels.Well I've also been dosing 2ml lanthanum chloride daily with no effect. Was also planning on using gfo.
As mentioned before, the binding and unbinding happens fairly quickly, and with .5ppm in solution there must be a boat load "bound" to the substrates.
Even better, take out a gallon of tank water and add 2mls and see if you get precipitation, and measurable lowering of PPM P in the gallon of water. That should help figure out if your LC treatment solution is working, or if it is too dilute for the issue you are trying to address?