I have a 20 gallon tank with 2 white skirt tetras 1 black skirt tetra and 1 red tail shark. The red tail shark chases the tetras and the black skirt tetra chases only the 2 white skirt tetras. I want to buy 2 more tetras so they can school but I am worried about the red tail shark stressing them out. The 2 white skirt tetras are also stressed from the shark that’s why there fins are ripped I looked at it and it doesn’t look like fun rot. I don’t know what to do about the red tail shark? Help! Please.