I don't think fish brains have the capacity to experience the emotion of boredom.... but I would agree that some of the more intelligent fish may become lethargic and anti-social due to lack of stimulus. ...kind of like some humans.
well i dont believe they get bored i mean my pleco loves to eat his algae on his ship and my bala sharks like to group in there favorite corner and they are very chilled out hopefully i can get a different tank with some oscars or something!!