Yes. Run a skimmer, also
I dilute 10 or 20 ml into 500 ml rodi and drip over 8 hours into overflow filtered with a 5 micron sock. Every couple of months. Do not use if po4 is below 0.2 ppm, you want all the lanthanum bound to po4, not your livestock. After the drip, run the sox another 12 hours.
I use this as required, if the po4 gets above 0.4. Also remove gfo before use, and replace with new after.
Do not lower po4 more than 0.5 ppm per 2-3 days
How much you add depends on po4 level. In my 75g, 20 ml will lower 0.5 to 0.1 ppm. It would scale linearly, so for 200g 50-60 ml for the same result.. I ran a test 1st with 10 ml to 'calibrate'.
I use a dosing pump to drip, but IV bags work too.