Even if the used fixer becomes harmless once dumped, its irresponsible to dump non-renewable, recoverable resources such as silver down the drain. Much better to recover the silver or donate used fixer to a photo lab who will recover it.
Where do you think the silver came from in the first place?
"Hate is an adolescent term used to stop discussion with people you disagree with. You can do better than that."
—'blanksy', December 13, 2013