Many thanks for your quick answer !
I am glad that this oxydation doesn't affect the Silver Magnet good function. However, I noticed the used fixer I stored have already a strong sulphur smell, when opening the storing container, and that odor persists during all the time I let the Magnet on, I hope I didn't burn the cathod !
About the deposit in the storing bottle, maybe you are right about the time, because the fixer stayed here several months.
BTW, I hope all is OK for you, I read in another thread you are in NYC.