There are oxidation products as fix dries that form sulfur and sulfur containing compounds complexed with silver.
The best solvent I know of is a strong solution of lye, but this fix is worse than the disease due to the nastiness of strong lye solutions.
It will work and even clean out the grunge from old hypo bottles, but beware, the lye solution is very dangerous and will burn flesh and blind eyes. It is insidious, as you sometimes don't even know you have splashed yourself.
I use rubber gloves, a lab coat and safety glasses with shields when I use it.
PE