Some ideas from our very own pets of the day....
http://petoftheday.com/archive/1999/May/08.html
http://petoftheday.com/archive/2000/August/22.html
http://petoftheday.com/archive/2002/November/10.html
http://petoftheday.com/archive/2002/July/07.html
http://petoftheday.com/archive/1999/April/09.html


or snakes...but if you have rats maybe that is sort of a conflict of interests. Some people like 'em, some don't.

....granted I haven't done any research as to whether these creatures fair well in aquariums. I'll let you do that part!