Interestingly, the same advertisement uses a different plural form of fish on each panel: If fish could talk ..., but Guess the number of fishes.
According to the Oxford Advanced Learner’s Dictionary, the usual plural form is fish. However, the old-fashioned (e.g. Biblical) form, fishes, may be used to mean varieties of fish.
To illustrate: there may be 20 fish swimming towards the tank; but if there are equal numbers of salmon, trout, plaice and halibut, then there are four fishes.
Perhaps Tetra should make it clear which sense of fish/fishes they intend ... or perhaps this contest is a test of one’s knowledge of grammar rules rather than one’s counting/guessing ability?