HOB means hang on back filter in case you are a little confused.
I just wanted to say that if you get a filter you should look up these threads about cycling which is the main job of the filter: to become a home for ammonia eating bacteria. That help you keep your tank cleaner, and maintain water quality you will do less water changes, but they are needed nonetheless.
A beginner's guide to cycling:it will help understand what is cycling and the methods
http://www.bettafish.com/showthread.php?t=47838
Step to step cycling by Olympia:
http://www.bettafish.com/showthread.php?t=111960
Betta specific cycling by OldFishLady
http://www.bettafish.com/showthread.php?t=107771
For cycling you will need a water testing kit as well.