When fish tanks stop being clean they start being smelly.
When fish tanks stop being clean your fish start being dead. Flushing a beloved family pet (who has been in the “family” longer than my wife) down the toilet is distressing. Even if it’s just a fish. Flushing two of his fishy brethren at the same time is even sadder.
When fish tanks are not cleaned regularly the pump gets all slimy and stops doing its job. When your pump stops doing its job your fish die and the clean up job is much bigger and more daunting.