I could argue that the stone gave him the ability to become a mermaid, which does not necessarily imply an ability to become human again.
The fundamental idea that a mermaid would be forced to be in an aquarium is incredibly unlikely. And could potentially create a good amount of drama that the business oriented aquarium is intentionally downplaying mermaid intelligence to sell tickets, bad press that could come down later be damned. Or maybe the story could be branched or extended so that Jon is actually kinda dumb now.
But at the end of the day the story idea that got me was that Jon is forced to be in a tank as a mermaid and face the embarrassment of it and the embarrassment of being treated like an animal.
As for Nathan, I thought of a few scenarios where he was dropped in the tank, or was "discovered" alongside Jon, but I thought the more interesting idea was that he was just there, with no (immediate) clue how he got there. I guess that's not technically within the stones rules, but it seemed more interesting that way. TBH, I'm not particularly concerned for giving an explanation of why he's there, just that he is.
