ive had my pleco for around 8 years since he was small he is now about 17" and is living in a 200g with some 6-7" cichlids an he has turned into a psycho he use to live with much larger fish an was never a problem, has he become aggressive due to the smaller size of his tank mates also i have a large piece of wood in the tank which i have recently put in could the smaller swimmin area cause aggression also. cheers guys