The rocks in your tanks are most likely to be limestone, put there to neutralize acidity. We had a darkroom built by the design team that turned an old athletic facility into an art center and they put those tanks in th enagative room and in the darkroom. Every few years ( supposed to be months, I think) we add some more limestone and ceremoniously seal up the tanks again. There hasn't been a sludge problem. I can't say it's useless, though our water is slightly basic, not acidic here in the 'burbs of Boston, but as far as I know it does squat for silver in the system. We're looking for a good solution ourselves. The "cart it to a hazardous waste facility" solution doesn't cut it for us and the town seems to have no problem with us dumping some fix in the sink. I'd just feel better if we reclaimed some silver before the flush.