[QUOTE=dancqu]Sorry, but...Originally Posted by Ole
The point is not the halogens - thiosulfate complexes very well with silver, not halogens. Same for ammonium, another silver complexer - but not as efficient. Ammonium also forms complexes with halogens, but to a much lesser extent. This increases the solubility of the silver halogens, of which sliver iodide is the least soluble. In this way ammonium thiosulfate (or ammonium and thiosulfate) dissolves silver iodide in a fraction of the time it would take without the ammonium. This lets silver iodide be fixed out in a reasonable time, which it won't without ammonium.
I'm afraid you've got the fixing process backwards: The halogen ions are very soluble, the point of fixer is to make the silver unavailable to them by wrapping the silver in tight complexes. And the solubility of the silver halogen compounds play a role in the speed with which this happens - (silver) chloride is more soluble than the bromide which is more soluble than the iodide. About a factor of 10 difference with each step.