First figure out which wall you want to use? Is the aquarium going to be viewable on one or both sides? Will you access the aquarium from the front or back?
With single-sided viewing and front access, typically you will have a picture frame style border with the top piece removable or hinged. With double sided viewing one or both sides will have trim with access under the top panel.
Baseboard trim, crown molding and wood boards all make good trim. Avoid MDF because it swells when wet. We sell pre made square and curved trim out of knotty pine here: http://www.walltanks.com/tank_4.php