Cichlids are territorial, but it makes them interesting in an aquarium. They do require some education as to which ones can cohabitate. I had a friend that had a ginormous Oscar. It recognized him and would come to him as he approached the aquarium. I figured it was because the Oscar was hand-fed.
I had some Red-belly Pacus when I lived in TX that got pretty large. They were made illegal and I ended up having to destroy them when I moved (I wanted to find them a new home). Turned out that they could reproduce in the native waters and become an invasive species. They were the closest thing I've ever had to piranha in my aquariums.
ETA: Invasive Red-bellied Pacu Caught in Lake Conroe
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