Thanks for all the info. You have answered a question thats been bugging me for a while. I had been buying acid at Home Depot for about $8 and been using about 1 gallon a week. I recently bought some at a pool store for slightly less and my usage dropped to 1 gall every 2 weeks. I found an old Home Depot carton and sure enough the acid was half the strength - Wow! HD acid was also a yellow color whereas the new one is clear.