Function of hypo used in fixer is:
An aqueous solution of ammonium thiosulfate (“hypo”) dissolves the unexposed silver halide grains. It forms stable, water-soluble complexes with silver ions, which diffuse from the emulsion. Excessive fixation (hours) results in a gradual loss of film density because the grains of silver slowly dissolve in the acetic acid of the fixing solution.
White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition