Ever wondered why Pentagon is called Pentagon? Unlike other buildings, it is a five-corner building and Penta is five (5) in Greek counting. Just like Penta, we have Tetra which means four (4). Now, this raises a new question about why some fish is called Tetra Fish. These fish carry square-shaped four-corner fins which compelled the Fish keepers to call them Tetra Fish. So basically, they are named after their fin shape. But, over time, many aquarists have taken negative leverage of this simple Geometry by wrongly adding many fish to the beautiful family of Tetra Fish. Once you go to buy Tetra Fish, you come across many species that are sold as Tetra Fish but actually, they are far from Tetras. That is why you may find the professionals addressing Tetras by their Biological family names.