I went to my local pool store to get some stuff I needed for opening my pool. I use liquid cholorine and they have pretty good prices for the 12% stuff. I also use tri-chlor pucks when I'm away or when I need to shock the pool to help keep my chlorine levels up. I also use them to raise my CYA, which is usually low when I open. I generally keep my CYA on the low side so I can throw in pucks if anything looks strange and not have to worry about it.
Anyway, they had two brands of tri-chlor pucks. One was $49.00 and the other was $64.00. Both were 25 lbd containers and both were 3" pucks. I checked the ingredients and both listed:
I asked what the difference was and the owner of the store told me the more expensive pucks were 100% chlroine and did not contain any "binders." She said the cheaper brand were imported and used a bonding agent that can cause a black ring around the liner.
I'm suspect of this because the ingredients appear to be exactly the same and its obvious the more expensive brand was not 100% chlorine, but I can't figure out why the one brand was so much more expensive if they are the same exact thing.