Water conditioners do NOT really remove heavy metals, other than trace amounts (in the ppb range) and what would typically be found in municipal drinking water. In other words, it's a bit of a gimmick when companies state that they remove heavy metals because in the case of a large heavy metal spike, say from a repair on a broken water line, your fish would not be protected from that large spike. The typical heavy metal ranges found in the vast majority of drinking water supplies is no different than what is typically found in nature. (lakes/rivers) There are exceptions of course.
I have a sticky on water conditioners in this folder, you might want to read it.
BTW - sodium thiosulfate is not the same chemical make up as the formulas that make up Seachem Prime/Safe - but from what I have read sodium thiosulfate also reduces 02 levels in the same manner as the Seachem products. Duane would be the one to chime in on that if I am mistaken.
I still suspect an overdose of the product, easy enough to do with reducing agents, especially if the 02 levels in the tank were already borderline low. As an example, I know a gal who once lost a tank full of adult cichlids because the air bubble wand in the tank quit in the night. I have never used an air bubble anything, but I also understock all of my tanks - her tank was heavily stocked, so much so that the loss of 02 exchange from that air bubble device, cause the death of all of her prized African cichlids while she was sleeping.
