Quite often I get $5 Leslie's coupons off $25 when you get a free water test (I know). NPS (Phoenix, AZ) has the chlorine 2 gal for $4.95. I price match NPS at Leslies (tell them to make the math simple and use $5 ea) to NPS and then stack the Leslie's coupon so I'm getting 10 gal for $20.
If you're really brave or have a lot of free time, you could pay the full Leslie's price $6.56, and then come back and take advantage of the 120% of the difference guarantee (6.56 x 4 - 5 - 7.92 = 13.32 for 8 gal) at $1.665/gal or $2.29 if you don't have a $5 off $25 coupon.
The 120% difference might be kind of touchy. I once asked the cashier if anyone ever pays full price, and then whips out the lower price ad to get the extra back rather than price match. He said, he doesn't have to give the extra 20%, and he'd never do it for me because I price match pretty much everything I buy there. So I won't try it at the store that I go to all the time, but I would be willing to try it at a different Leslie's the next time I'm near one.
They happen to have the coupon on their website under "store locator" that's good until 7/5 if you don't get coupons emailed to you. Actually, this doesn't have the $25 requirement.