Very different fish--different families, even. The sailfin tang has the knives on it's cauduls, can collapse the large dorsal fin, and is primarily an herbivore. Bats (and there are several species--the orbitculus is the most often seen) get about the same size, but as an adult are shaped much more like a large angel, and can not collapse their fins much. They need a tall, large tank, don't swim around as much as the tangs, and prefer a meatier diet. The juvenile bats may look similar to a sailfin in shape and color, but most are an orange brown, and look more like a leaf. They are in mangrove and treed coastal areas as juveniles, then move out to the open sand and reefs as adults.