Unlike the notorious Sea Monkeys, this product from Wham-O was not falsely advertised. They found a species of fish, the African Killifish, which laid eggs in mud which could remain dormant until re-hydrated.

The problem was that once in captivity, Wham-O could not get the fish to breed enough to make a sufficient supply of eggs for their product. They did sell a few units of Instant Fish, so it was very briefly a real product, and it was as the box depicted. They pulled it from the market due to being unable to create enough eggs.

This is an African Killifish. Compare to the box art fish in the aquarium, and they did depict more or less what you were going to get.