Not to thread-hijack, but let's pretend I would like to remove my iron-sequestered water with fresh, trucked in water one day. My liner is vinyl and I've got 22,000 gallons. Is the tarp method a "safe" way for me to drain/refill too? Eg. maybe 8 - 10k gal at a time?
At any rate, to Guinness -- You didn't mention whether or not your water is clear, or if you have any signs of algae. For a 15,000 gallon pool, 1 gallon of bleach is going to be roughly 3.3 ppm, right? (Is that what the pool calc says? My math isn't so hot). I might be tempted to add two, even though you're cycling out the water. 7 ppm is technically the MINIMUM sanitizer if your CYA is 100 (which is as high as our chart in pool school goes) so if you want to be a minimum chl level, I suspect you'd need two and a bit. Now, in actual fact, you ARE going to be wasting most of that via draining...but I'm just thinking if you don't want any algae forming (and if it hasn't already started to form) you might do that preventatively. If you're going to need to shock anyway....then I suppose there's no real need to stay at min.