For me, the 5 gal carboy of the higher concentration bleach is by far the most economical. The intial deposit is $6 and refills are $3/gal. I keep the carboy in the garage and transfer the bleach to an old Clorox bottle with a syphon.

Not only do I save a little bit of money but I cannot justify the large number of plastic bottles I'd be going through if I bought bleach by the gallon. It's a pretty serious waste disposal issue out here in the contry.

I buy the carboy refills at the pool store. Pool stores aren't all bad

Anna