Plain water softener pellets, solar salt, or pool salt with a purity of at least 99.5% are fine. Just pour it in the pool and brush to dissolve it. It actually dissolves pretty fast. I have a SWG and have used all of the above. Pellets take the longest to dissolve, pool salt (smaller crystals than solar) is the fastest. Two grocery store chains in my area carry pool salt (Publix and Winn Dixie) and have the best price that I have been able to find (about $5.00/40 lb bag).