Maybe it's doing it out of hunger.Feed it a little extra.This should stop it nipping the others..but if this doesnt work, make sure it has good living conditions. Is the tank large enough to support them all? You might need a bigger tank- it could be looking for more territory or better housing.However, if it already has a good enough tank, then it could just be aggressive by nature.If this is the case you'll have to separate it from the other fish.hope this helps.