I doubt it is toxic at all, otherwise you wouldn't be able to use it in the tank in the first place. If it sits in the tank, the biggest danger would be from it leaching the toxins it's taken up over time. I would look to a different cause of the bleaching/RTN
. If the digi was moved drastically higher in the tank, that could be the cause, temp spike could be the cause, and sometimes we don't know what the cause was, it's just coincidence that you noticed the bag empty. More than likely it was empty within a few hours of being placed in the sump.