What is a pet fish mean?

I have a LWH 48x13x21 (55 gallons), and I want an aggressive high cold, fresh water fish that is easy available and could live comfortably in the aquarium thanks ~ ~

You can get an Oscar (one and only one). To be quite large (about 12 to 16 inches) and are aggressive. You can get at most pet stores you can even get the albino type. Here is a photo> http://www.flower-horn.de/Oscar_Data/Ocellatus.jpg Here is a site to learn how to care for them> http://www. Firsttankguide.net / oscar.php They are very good looking fish! I hope you find something you like and good luck!