Help! I'm a Fish/WMG

Fly is a Time Lord.
His hat is his TARDIS. Sure explains his, doesn't it?

The fish potion has no time limit.
Or at least it has a longer one than Mac Krill said. How would he know? Has he ever been stuck as a fish forever? (Remember, forever is a long time.)

Also, watches rarely run true, especially ones that have been submerged under water for a long time. Chuck's watch was probably behind by a few minutes or seconds, and so their "time" ran out earlier.
 * I now realize that the potion may actually have a time limit, as Mac Krill may have had a few "test subjects"...

Joe
If this movie ever has a sequel (though it is highly unlikely), then it would be built around this concept.
 * But

Joe and Professor Snape are the same person.
They both have the same voice, and they are both obsessed with potions.

So, after Joe, he gained magical powers (an after-effect of the potion) and later found a remote place which no one knows about -- The Wizarding World. He wanted to be evil, so he. He eventually ended up as the potions master at Hogwarts.

Let's vice-versa that. Maybe Snape was so Crazy Prepared that he had a Turn-Into-A-Fish Potion on him and drank it at the end of Harry Potter and then became a fish.

Sasha is one of Professor Mac Krill's failed test subjects.
Originally, after testing the potion on a person, he found that it was too strong and caused the loss of all human intelligence. Sasha was lost in the same way Stella was; by the time Mac Krill found her, it was too late. She was stuck in that form forever.

The US cover has such god-awful art because the US distributor didn't want it to be a success.
Some idiots probably didn't want a non-Disney (or Fox, DreamWorks, WB, whatever) and non-anime animated film to have much success in the States. Since CG-animation was popular at the time, they made a CG cover and made it look deliberately terrible to keep people from buying it.
 * Must be the same guys who deleted many useful scenes from The Fearless Four.

The US cover was a huge Take That toward American animation of the era.
This terrible cover was essentially mocking the States' obsession with CG animation over traditional animation. This cover art was also a test to see if Americans wouldn't judge a book by its cover.

This movie was originally going to be part of a six-part anthology corresponding with Merlin's tests in The Sword In The Stone.
We know that in The Sword In The Stone, Merlin tests Arthur by transforming him into various animals, the first of which is a fish. This movie revolves around three kids who get turned into fish. This could mean that a series was planned; each title would be ''Help! I'm a/an'' [Insert animal name here], and each film would have a different group of kids getting turned into the eponymous animal. It was going to be a combination of tests from the novel version and from the Disney movie. However, this idea never took off, as the movie didn't get a lot of publicity, and the creators feared it would turn into a Franchise Zombie from all the sequels. So, they just abandoned the plans.