Once they settle in they can get even more territorial then before. I am sorry but I think the reason for the aggression is that the tank is too small for him so he just decided to declare the whole tank as his own territory and no one else is welcome. I think he needs to go back to the store and then try to get some fish that are better suited for your tank. Perhaps a small school of tetras. I like the rummynose tetras and the lemon tetras are also nice. Both of these species will school and keep the tank active with plenty to watch. Just make sure you go with one or the other. Another species to consider would be the rasboras. I like the harlequin rasboras. They look like they are wearing little tuxedos. They are tight schoolers and look very nice swimming through the plants.
Hope this helps.
Marinemom
Hope this helps.
Marinemom