Zander (Sander lucioperca), perfect to be held in an aquarium, though once it reaches 22cm and eats 5 goldfish a day, better make sure you have a few of those ready and steady, which illustrates the only problem i had with my pet Glasoog, which therefore had to be released in the wild. The only other things to remember are that it highly depends on the right type of water, and will attack anything that enters the aquarium.