Fixer removes the unexposed silver halide remaining on a hologram leaving behind the reduced metallic silver that forms the image. By removing the unexposed silver halide, the fixer prevents any further reaction of the silver salts and ensures a permanent image.
Most fixers are based on the thiosulfate ion, especially ammonium thiosulfate. Sodium thiosulfate or 'hypo' was the commonly used fixer. Both fixers work best in acid conditions and this is usually created using small quantities of acetic acid.