I've been using Kem-Tek muriatic acid for several years. This year they changed the design of the cartons holding two 1 gallon jugs of the stuff. Recently I noticed that after adding acid according to numbers from The Pool Calculator, my pH was still running high. While at a local Home Depot yesterday I looked more carefully at the cartons. All were stored on the same rack, and all were the same price. I was shocked!
One style Kem-Tek carton, with a generic designation for "pools and spas" showed HCl concentration: 14.5%
Another style Kem-Tek carton, labelled "Contractor Use" had HCl concentration: 29%
Another plain brown carton, no manufacturer name apparent, listed HCl: 31.5%!!!
I believe I have been inadvertently using the watered down 14.5% solution for several months, and am quite sure all the cartons last season had 31.5% concentration. Needless to say, I bought the carton with the 31.5% HCL at the same price. I've not yet spoken to the store manager about this, but plan to do so.
So, be sure to look at the composition of the chemicals you buy, even if it looks like a product you are familiar with, and have used in the past. Caveat emptor!