There are several different types of freshwater aquarium water conditioner promoted as being specifically for Bettas (or Siamese fighting fish). However, this water conditioner is also safe for goldfish, another freshwater species.
Ingredients
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Water conditioners for freshwater fish all act to help remove chlorine, ammonia and other toxins from tap water. According to Aqua-Fish.net, all water conditioners are made up of similar ingredients.
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Testing
The makers of Hikari Betta Ultimate Water Conditioner urge fish owners to test their tap water or current tank water for ammonia before adding in any conditioner because the conditioner may not be necessary. They recommend kits listed as the "salicylate" type.
Disadvantage
Betta water conditioner may not be convenient for goldfish owners to use because they come in such small bottles. One dose may use up the bottle for a large goldfish (or Betta) tank.
Misconception
"Freshwater Aquarium Problem Solver" (David E. Boruchowitz: 2006) reminds all freshwater fish owners that no conditioner will substitute for regular partial water changes in keeping good water quality in a tank or bowl.
Warning
"Freshwater Aquarium Problem Solver" also notes that a goldfish tank should be set up and running with conditioned water for a month before adding any fish. This way, friendly bacteria have a chance to grow and help to keep the water balanced enough to support fish.