An aquarium may be of adequate volume to house a large fish such as a tang, but it’s not just the amount of water you need to be concerned with. Large fish need long distances to swim. Long, low, and wide aquariums are best for these fish, as they provide the maximum amount of swimming room. Tall and narrow tanks just simply do not provided the swimming space for large fish, and are better suited for smaller reef fish instead. I personally prefer a large aquarium with a big footprint, and that’s the direction most people are moving in anyways.