Our 50 lb. Sure Soft Extra Course Solar Crystals are 99.9% pure. Salt purchased at big box stores is not as pure, which means they contain dirt and sediment along with the salt. We recommend letting your brine tank run empty of salt once per year so it can be cleaned out (if you are using our salt). With cheaper salt, you need to clean it every 6 months. Give us a call. We will deliver the salt and put it in the brine tank for you at no extra charge!