The point is not to reduce the LaCl. It binds to PO4 and precipitates out, hence why you get all of the gunk on the sides. As phosphates leach out, they bind to the LaCl and form a solid, which can be brushed off. By changing the water, you are reducing PO4, which will cause more PO4 to leach out, but you'really also reducing the ability of the LaCl to reduce PO4, because you're removing the LaCl with the water change. If all of the LaCl has precipitated out of the water, then you're just reducing PO4 by doing a water change. The goal is to have the LaCl do the work instead of a water change though.