In 20000 gallons, 5lbs of granular CYA will all about 30ppm. It takes CYA a long time to dissolve, you can pour it slowly into the skimmer and wait for a few days or you can put it in an old sock and hang that in front of a return jet - squeeze the sock now and again to help it along. Be sure the CYA is pure Cyanuric Acid - 99.x% with no other active ingredients.