Why is my black moor just sitting there hiding?

i added a black moor along with the 2 i already had.and now it just hides and just sits there.it is the middle fish in size now.its not upside down or nose diving .when it does swim it moves perfectly fine.it is eating and swims fine when its feeding time.the other 2 fish are perfectly fine and my tank is plenty big enough and i do my water changes on cue.the water quality is also good just checked it before i asked this.there are no spots on it and doesnt look sick as it is eating and swimming fine when i feed them after it eats it just goes to the back of the aquarium and hides and doesnt come out