I think Twig herself acknowledges that there may have been a better course of action than what she took, when she said she "lost it".
Not only was the "fish guy" (owner or employee) mishandling the fish, AND misinforming or failing to inform his customers, he was also Rude to twig who was in fact a customer. Perhaps there are legitimate reasons for not letting a customer scoop out her own fish, I don't know. But there are certainly Appropriate and Inappropriate ways to deal with customers. I have had encounters with "fish guys" who are rude and insulting on top of being wrong, and it is pretty infuriating. (I know there are hard- to -deal- with customers, but that is no excuse.)
I fairly regularly share appropriate information with customers in fish stores when for instance they are about to buy a GF for their 5 G or similar. I try to do this in a polite manor that is respectful to the employee involved if there is one. Some employees are appreciative, some apologetic, and some are just ***holes, as the one in twigs case seemed to be.
Frankly, while it may be legal, I don't feel anyone has a right to make money mistreating fish any more than they do by mistrating any other animal, or by selling defective merchandise for that matter. I would interfere with that sort of "business" without qualms any day.
I can't resist adding that while people can bring suit for anything or nothing at all, and in this country too often do,there is no cause for slander action implied in the story of the OP.