I have a 20,000 fiber glass pool and need to drain it half way or all the way, when I tested the Cyanuric Acid levels it was around 140. I had to add 6 packs of shock just to keep it blue every other week. Now I am trying to drain it and was wondering once I add the new water which is added from the same hose I use to water the trees, how much Cyan-uric Acid should I add to the skimmer? They have pumps at Home Depot which I will use, but how would it work? Because once I drop it in and plug it in to the outlet isn't dangerous?

Location is Southern California.