If you plan on adding a yellow tang or two....I just got two nickel sized yellow tangs today..It will take some time for them to grow out so a 55 gallon will be sufficient. At that size the fish has a high metabolism so they will need a sufficient amount of algae present in your tank...I would recommend feeding them twice a day as well brine,spirulina....and putting some lettuce on a veggie clip....aloha Les