I understand your predicament w.r.t. the storage of the water, but if you look at the numbers, it's pretty compelling to make the biggest water change you can:
At 80 ppm
, a single 50% change gets you to 40. Another 50% drops you to 20.
With a sequence of back to back 10% changes, you get to 72, 65, 58, 52, 47, 43, 38, 34, 31, 28, 25, 23, and then finally 20. Even just to get to the 40-range, it takes 6 PWCs at 10%.
Again, I completely empathize with you w.r.t. the logistics. If you have an LFS
that might be able to work with you on it, or a local club with some members that could lend a hand, I'm sure they would all be willing to help.