sodium thiosulfate doesn't work as well for chloramine as chlorine. chloramine is ammonia added to chlorine. there's a few different types of chloramine but from what i understand, when you use sodium thiosulfate on chloramine it breaks the ammonia off from the chlorine, then the chlorine gets broken down as normal, but you end up with a lot of ammonia in the water, which is obviously also deadly to fish.
chlorine will off gas from water in about 2 days. a fella at the fish store told me chloramine takes 42 days to completely dissappear (is he exagerating?).
a question I have though is... if you hit chloramine with sodium thiosulfate and you end up with the ammonia in your water, how long does it take ammonia to off gas? or does it?
the dechlorinatores that take care of chloramine aren't usually sodium thiosulfate and they end up binding the ammonia up in some fish safe way as well.
i've researched this quit a bit because i'm raising fish to eat and i try not to use any chemicals. you should be able to google your cities water utility and see what they use, or maybe give them a call.