Considering their kids first went through the radical process of turning into fish as unplanned, I think we can all agree that the parents would demand to have a sip of the fish potion as well, so that they can all relate to their gill-bearing children & support them while being all scales & fins themselves.

The professor would not object to this since he originally intended to be his own guinea pig, though he wouldn't expect to come out as a microscopic crustacean individual, despite literally being named after those species.

ANYWAY NOW THE ENTIRE FAMILY'S A FISH! :D (or other marine creatures in general but shut up)