Bob the Goldfish is a Character on DEXTER. He was a pet belonging to Cody Bennett, who eventually died and Dexter Morgan stood by Cody as he dropped the fish into a toilet to flush it. Cody remarked on wanting a "snail now", before walking off while Dexter did a monologue on the fact that he's sent many to a watery grave. As he says this, the fish somehow comes back up from the toilet which symbolizes a later part of the episode where Valerie Castillo, a recent victim, emerges exactly where he killed her despite dumping her into the sea.
Regardless, Dexter flushes the fish again and later deals with any evidence that could link him to Valerie's murder.