Hmmm, that's sort of a tough call. A WAG on bleach costs would probably be $40-$60 and you'd still have that really high CYA as a lingering problem.
If 5000 gal of city water was reasonably priced, I'd drain 1/2 of it. A 2/3 drain would be even better. You'll still have to use quite a bit of bleach (to get rid of the algae/green water) but most likely not as much and you'll have gotten your CYA manageable.
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