One gallon 12.5% chlorine is essentially the same as 2 gallons of 6% bleach. I believe some one on here once mentioned you may want to stay away from the generic bleach at Wal-mart. To many unknowns in the bleach, may not even be 6%. The 12.5% bleach will break down faster and will not stay true 12.5% as long as 6% stays 6%.

I have had pretty good luck with 12.5% pool shock from BJ's. Its a local big box store here. Like a Sams club or Costco. I am about to take on my neighbors pool with the stuff. Their pool is very very green almost black. They just opened it for the summer and said they spend $1000 opening it each year then have to shock it all summer to just keep it somewhat clear.

Shipping is cost prohibitive for bleach over the internet. Too bulky.

I switched last year and my brother switched end of last season, we are both believers now.