Approximately one out of three homes owns, or has owned, fish as pets. A large percentage of murders occur within the home. The police have discovered a way to communicate with fish to obtain leads in investigations. This interrogation technique is kept a closely guarded secret (oops... sorry for telling) because it keeps people from killing the fish and information obtained isn't legally admissible in court due to the fish being unable to request lawyers and the fact that courts currently do not issue subpoenas for sea life, and if the fish is unable to testify in court, its testimony is automatically considered unreliable. Regardless, because criminals rarely are aware that the fish can rat them out, the witness remains unmolested and is able to share information with the authorities. This is where the lie becomes important. Even if criminals do discover that cops can communicate with fish, the police-perpetuated myth that fish have no memory convinces any criminals who do know about interspecies communication that attacking the aquatic pets is pointless as the fish will forget the whole event the moment the crook steps out the door. Once more, the witness lives.
And I know, you're all asking why I'm sharing it here if it's such a big important secret. Simple. No one goes around killing fish because Aquaman can talk to them, and he's a superhero. Crooks are lazy. They want to do things the easy way. Otherwise, they'd get real jobs and and earn their money the honest way. If they aren't concerned about protecting themselves from the Justice League, they certainly aren't worried about the cops.
You have been informed.