Why do you have him in a water? Do i read it right?

You gotta do a qt tank for him and watch closely, he might be having more of these pebbles. Also you need to go regular qt route with fecal, etc.
did they say what he was eating? I would try to feed him nightcrawlers only ( if he will eat them) and do honey baths in case he has more stuff to get rid off.

But seriously though - plastic fish bag? Oh boy....