The addition of an ammonium salt to a sodium thiosulfate fixing bath results in an increase in the rate of removal of silver halide from an emulsion. HOWEVER, the rate is slower than a comparable ammonium thiosulfate bath. The "unwanted" sodium, chloride or sulfate ions slows the rate.
Discharge of ammonium salts into water supplies is a problem since the ammonium ions are toxic to fish. There is at least one patent which states that 60 mole percent of the ammonium thiosulfate can be replaced with sodium thiosulfate. However, I could find no discussion as to the efffect on the fixing speed of the resulting mixture.
Kodak suggested the use of ammonium sulfate instead of ammonium chlroide which it is corrosive to some stainless steels. However the ammonium sulfate containing fixer is less effective in its action.