This actually addresses a question I had about this. You say with the 12.5% if it gets warm or if its in the sun it will "go bad".
From reading various things here, I thought that the higher the concentration and the higher the temp/sun, the shorter the half life. So, your 12.5% bleach, if subjected to high temps/sun with drop to 6% in a month or so. But, then shouldn't the half life now be the same as 6% bleach? Or does it not quite work this way?
Does the blanket statement of "going bad" really just mean the the actual chlorine level will likely be less than the sticker, but still "good" in the sense that there is nothing wrong with it other than having a lower content and not "really" knowing what levels you are adding to your pool?
Although given this quick drop and not knowing how the store handled/stored it, is the fear that you may be buying something with no more chlorine in it than you would have gotten in 6% bleach for less money?