OK. If you can, keep track of it.

I'm beginning to think that the ratings are 'ideal', not real. Adding lanthanum chloride to a pool can form lanthanum carbonate -- which is ALSO rather insoluble.

I believe that, over time, if the lanthanum carbonate remains IN the pool (pool floor or on filter media), it will be converted to lanthanum phosphate. But if you clean the filter before that happens, you'll end up having used a portion of your PO4 remover as an expensive and inefficient way to lower carbonate alkalinity!