The best replacement for thiosulfate is sodium sulfite.

The best alternatives to thiosulfate are rated as following:
★★★★★  sodium sulfite
★★★★  ammonium sulfate
★★★  sodium bisulfite
★★  sodium metabisulfite
★★  thiosulfate
★  sodium thiosulfate pentahydrate

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Speaking about thiosulfate, one probably shouldn't forget about triiodide.

Other search terms related to thiosulfate and their ratings are:

★★★★★ triiodide
★★★★★ iodine
★★★ iodate
★★★ kio3
★★ chlorine
★★ hypochlorite
★★ nitroprusside
potassium iodate
iodine reaction
You have in mind a companion for thiosulfate and it is not yet here? Propose!

Google Trends

Scale is based on the average worldwide traffic of thiosulfate in all years. Click on graph for more.
sodium sulfite
ammonium sulfate