It depends on what you consider "monster." To make my "monster" list, the species has to meet at LEAST one of the following criteria: big, mean, or odd. This puts you at an immediate disadvantage. "Big" doesn't work with a 30g. "Mean" usually denotes eating and/or killing your algae clean-up crew, which isn't completely necessary to have, but it definitely helps. This leaves "odd" and IMO, your best bet of finding a "monster." There are tons of fish that are odd enough to be considered monsters. I've always thought that african butterfly fish were pretty neat, and I think they'd do okay in a 30g.