There are several fish that serve vital roles for our slices of the reef from algae control to pest control/eradication. For my tanks I must have a tang for algae (bristletooth or zebrasoma), some sort of wrasse for coral pests (six-line or one of the Halichoeres), and a Molly Miller Blenny for aiptasia (has always worked for my tanks). What are some fish that you'll always have due to the benefit they provide to the tank?