I have an abalone in my 10 gal - don't see him too often, but he is very cool! My other favorite is a vermitid snail - they attach their shell to something solid, usually the rock, and build their shell up in a cylindar. Fun to watch them go fishing with their mucus strands. For sure get a sexy shrimp! Also a porcelain or anenome crab - they have those nifty fans to catch food.