I recently purchased a 50# bag of stock salt at Tractor Supply. <$5.00 for 50#. It seemed to work well for mixing in the water to transport fish. But I realize that not everyone will need that volume of salt.
I don't know if it makes any difference to the fish, but the Stockmans salt at our local TSCs incluldes Yellow Prussiate of Soda (YPS).
I strongly recommend not using it for swimming pools. I learned that the hard way.
