"Rodrick's Fish" was a piranha-like fish that Rodrick bought.
In the book version, after Greg's birthday, Susan took the boys to buy pets. Greg bought an angelfish, while Rodrick bought an aggressive fish. Manny bought fish food and ate half of it. The fish were living in different bowls until algae grew in Rodrick's, so Susan put it in Greg's clean bowl. Rodrick's fish attacked Greg's and ate its body parts. Later Frank flushed it down the toilet but Greg believes it was still alive as he saw it moving before.
In the online version, after Greg's birthday, Charlie Heffley took them to buy pets. Rodrick was buying 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 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. Then Rodrick's fish survived on the algae in the water, when it was sleeping Frank, according to Greg's theory, flushed it down the toilet, fibbing that it died of natural causes but that might not have been so.