you're right on ... 20lb buckets of sodium thiosulfate used to be sold in swimming pool and spa shops ...
it is / was used to reduce as a "shock" against the chlorine. a lot of people used to
get a bucket to last a lifetime for fixer ... but not many people sell it anymore ( maybe something better is available ? )
...i have found 1 place in my area, but i would have to get 50lbs of it ( still for 20$ its not bad ) but i don't think my lifetime
will last that long .... ammonium thiosulfate ... from what i remember is sold as fertilizer ...
i wonder if someone had a huge gathering and everyone is clad in silver jewelry + armor if the pool-owners are permitted to just drain their pool
or if enough silver has been dissolved by the thiosulfate in the pool water that they have to have all the pool contents hauled away as toxic waste ?
considering it costs about 100$ for a waste hauler to remove 15 gallons of spent fixer ... it must cost a small fortune to haul away a swimming pool.