The Yellow Longnose Butterflyfish will reach a length of nine inches, is occasionally "reef safe", and should be kept in an aquarium larger than 125 gallons. This Butterflyfish is typically aggressive towards other relative marine fish species, and should be added after having "cycled" a new aquarium.
This species is relatively large by reef fish standards. They reach about 8.7 inches in length at full maturity and since they are active swimmers, the minimum tank size should be about 75-gallons (in my estimation). If you want to add in more than one Yellow Longnose Butterflyfish, you will need to consider buying a 100-gallon aquarium to allow the fish enough space to swim around.
The longnose butterflyfish is a moderately peaceful community fish that can be kept with most of the other peaceful, non-aggressive or non-predatorial reef aquarium fish. However, as mentioned earlier, they will be aggressive towards other longnose butterflies or even some other competing butterflies. The amount of aggression you could and should expect will also go up depending on how long these fish are in your tank. That’s not an uncommon phenomenon in reef keeping. The fishes who have been there the longest often become the most aggressive towards newcomers.