Look at the msds for toner and fixer. With the caveat that a lot depends on dilution (the amount of chemicals relative to the amount of other liquid in the system), I'd be leery of putting this in my septic tank and I wouldn't use it for any edible plant irrigation. Perhaps I'm being overcautious.

I realize you've been using a septic tank for 12 years, but I would note that a septic system can fail biologically and you wouldn't know it unless your soil was tested. If there's 2 of you in the home and you're in the darkroom every evening, I would be concerned. If it's 4 and you're in there once a week, then maybe not.