The simplest practical fixing bath consists of two chemicals; sodium or ammonium thiosulfate and sodium sulfite. The bath does not have to be acidic if a acid stopbath is used first. However, if a hardening agent such as alum is to be incorporated then the bath must be made acidic before the addition of the alum. The bath may be acidified in several ways. The most common is with acidic acid but sodium bisulfite or potassium metabisulfite can also be used. The acidifying agent must be well diluted and introduced slowly with stirring into a cold thiosulfate solution. It is tricky to make a bath without additional sodium sulfite when using a bisulfite. The solution should contain sufficient sulfite ion before the acid ingredient is added.
BTW, MSDS forms are not always completley accurate and their interpretation often requires some knowledge of chemistry.
Ansel Adams mentions the formula for the a two ingredient bath in The Print.