The Seaquel (My Big Fat Zombie Goldfish #2) by Mo O'Hara

When Tom's big brother decides to become an Evil Scientist, his first experiment involves dunking Frankie the goldfish into toxic green gunk. Tom knows that there is only one thing to do: Zap the fish with a battery and bring him back to life! But there's something weird about the new Frankie. He's now a BIG FAT ZOMBIE GOLDFISH with hypnotic powers . . . and he's out for revenge!