Poor fishy
Most likely your boy will okay, possibly an upset stomach or diarrhea from drinking the water. I did want to warn you though about the toilet.....make sure the lid is kept down! A YT member's yorkie recently died after falling into a toilet.
I would take him out of the bathroom, shut the door and hopefully soon he will forget and eat/drink for you. How long has this been going on, you didn't really mention that?