The Yellow Longnose Butterflyfish is known for its somewhat eccentric behaviour in the aquarium; it is for instance fond of swimming and hanging upside down in the water. It can also spit out water towards your at the surface, like a tiny fountain.
It should be mentioned that these fishes display some unusual behavior. Don't be unduly surprised should you catch yours swimming or hanging upside down; or that it might "spit" water in your direction at the surface. Also, let me mention their blanched whitish appearance on being exposed to light from dark conditions; like sleep or removal from a shipping box. A loss of yellow during the day is a fast sign that you need to be looking for a cause; probably fright from bullying, or diminished water quality.