flyweed, that's a great idea! I'm stealing that next time something like this comes up.
To add to the subject of "which bleach" to buy, as others have said, any brand will do. What you're looking for is plain bleach. No fillers, scents, additives, or anything special. You'll want to note the concentration, or percent of sodium hypochlorite. Most store brand bleaches are 6%.
Some Wal-Marts have recently come out with a 128oz size Great Value Bleach for the same cost as their old 96oz container. Look for the bottles that say "30% more free". It's right around $0.22-.23/oz of available chlorine or so.
Aldi has 96oz bottles for $1.19 usually...after taxes, it's usually down in the $0.22/oz range as well.
The math for figuring out your cost/oz of chlorine is this:
Price / (Qty * Pct NaOCL)
So, for example, the Aldi bleach at $1.19, for 96 oz of 6% chlorine would be:
$1.19 / (96 * .06) = $1.19/5.76 = $0.21
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Have no fear...You may well have that pool sparkling clear in time for memorial day! You'll get there!