The main issue is used/exhausted fixer. You see, silver is a natural anti-biotic, and too much silver over time can kill off the bacteria in your sepic tank, which prevents your tank from doing it's job.
There are various ways to remove silver from used fix - the old steel wool trick as talked about before is possibly the most simple and direct method.
Remember also that septic systems often get K'Od by people simply dumping too much used cleaning fluids down the drain. For example, you do a real, heavy duty spring cleaning some year, and after you finish, you dump a few buckets of used MR. Clean, Pinesol, Spic N Span, etc, down the drain, that is more likely to kill off a septic system than a few litres of used fix.
If your bacteria does die off, there are re-activators you can buy in local (mostly rural) hardware stores, but the old "cure" for a dead septic system was to throw in a road kill, preferably one that has been in the sun or heat for a couple days and a little smelly . Yeah, I know, gross, but I am told by both family and friends, nothing works as good.
You know, there are days I am very happy I am on a municipal water system.
joe