Imo, you should be dosing it either into a filter sock (some people suggest 10 microns, which is tough to do) or into the intake of your skimmer. Both Foz Down and Brightwell Aquatics Phosphate Destroyer are marketed for this use. Neither say their product is Lanthanum chloride, but both products work just like Lanthanum chloride, and there are many people that simply call it such.
Also, have you considered regenerating GFO?
http://www.advancedaquarist.com/2010/10/chemistry