In the Book version, after Greg's birthday Susan took the boys to buy pets. Greg bought an angelfish, while Rodrick an agressive one and Manny bought fish food and ate half of it. When they got it, they were in different bowls until Rodrick's fish bowl started to become covered in algae, so they merged his fish with Greg's. Rodrick's fish later ate Greg's fish awhole.
In the online version, after Greg's birthday, Charlie Heffley took them to buy pets. Rodrick was attempting to buy a snake, while Greg and Manny fishes, but Susan told Rodrick it was dangerous and forbade him. So Rodrick took an aggressive fish while Greg got an Angelfish while Manny a goldfish. First Rodrick's Fish ate Manny's Goldfish. Then it ate Greg's fish's eyes and soon ate it.