I am sorry to say, don't trust the person trying to sell you something to give you completely accurate information, doubly when it helps them sell the thing, especially when it is what you want to hear not what is probably best practice. I have been guilty on both sides of that equation, so totally understandable mistake. A 50 tall is a really small footprint for any butterflyfish long term. Difficult to balance adequate hiding/foraging space and open maneuvering space, in that size tank. I am not even sure the much smaller atlantic long snout or burgess would be appropriate for that size tank long term. The short nose yellow long nose butterflies, F. flavissimus, get 9" the far less common true long nose butterflies, F. longirostris, get 10-25% longer overall. both way big for a 50 tall. I personally would prefer a 120gal 4ftX2ft footprint as a minimum for long term housing of either species, I could see arguments for slightly in either direction. Butterflies need a balance of open space as not to feel cramped but lots of rock work to pick over in search of snacks, and plenty of hiding spaces to feel safe. Smaller individuals may give you some time to upgrade, but I would recommend beginning that process immediately if you plan to keep yours. It is my experience, young butterflyfish grow quickly. The Forcipiger are quite large at settlement, they have very long larval periods that have enabled their circumtropical distribution. This corresponds to a large minimum size, they simply don't come in tiny like other butterflies, angels and tangs.
Some general sensitive butterfly advice I would recommend feeding frequently with many different options. No aggressive tank mates, or overly boisterous feeders. Ensure a mature tank with lots of microfauna, worms and amphipods, to snack on between meals. A variety of caves of appropriate size to feel safe to sleep in, but plenty of open space such that they don't scrape their sensitive noses trying to get around. When I worked in a fish store, I dealt with F. flavissimus quite a bit, in store and client tanks; they are moderately difficult fish in my experience. Difficult to condition to captivity, susceptible to shipping damage, and disease. They benefit from frequent feedings but tend to be slow eaters, not always able to keep up with more aggressive feeders, so care must be taken to ensure they get some food at each feeding. Some people use feeders like PVC pipe with holes drilled in to allow the long noses access to food before the other fish in the tank, though not all take to such feeders. If yours is eating well thats a big part of the battle. F. flavissimus tend to be less reef safe than the Chelmon butterflies in my experience, though a touch easier than the notoriously difficult copperband. The long, long noses F. longirostris are incredibly difficult, and almost always come in with mouth damage. I have only seen a few, and the far fewer that had been in captivity long term were in the hands of some incredibly talented and experienced aquarists.
