Liquid, Powder, Don't matter (for those who wonder) even the brand is not all that important
Yes, there are many different "brands" of water softeners; however, be careful as one is not always as good as another. Softeners fall into either one of two "groups": Precipitating and non-precipitating, and most of the time, the labels will not tell you which is which. You have to know for your self.
The first group (precipitating) is not recommended for the GEO method as it has some characteristics that while are not harmful to our black tanks, does not really work as well. This group contains the brands: Arm and Hammer Washing Soda, Raindrops, Blue Dew, Borax, Climalene, Melo, White King Water Softener, and Borateem.
The second group (non-precipitating) is the one that is best used for the GEO Method and the brands are: Calgon (a mixture of non-precipitating and precipitating chemicals), White Rain, Blue Raindrops, and Spring Rain. These softeners ties up hardness minerals in the water and holds them in solution (sequestering). No visible particles are formed. The water remains clear and usually contains phosphates. The water feels slippery (which also makes the sides of the black tanks slippery).
So, when you can't find Calgon, look for the others....just make sure you don't buy the precipitating type. Also, it's much easier on the tanks (and you) if you only use a liquid for both the water softener and the detergent. I get my Calgon from drugstore.com in 32oz bottles (usually 4 at a time). If you "have" to use a powdered form, dilute it in a gallon of "hot" water and make sure it dissolves completely prior to putting it in your tanks.
Hope this alleviates some of the confusion.
...this is great information: thanks!