What would you do???fish help please

So we had 6 fishes (2 different types..)

Got them 2 weeks ago they been fine we just added 4 more... same breed.. same place ... one of the fishes from my old ones is attacking the new ones and old ones what would you do???

Its not the size of the tank we have a 40gallon tank. The person at the store said we could add more later on the same fishes and it was the same person who we got it from last time.... anyways should I take it out and give it back the old one its only one fish that is causing this issue I dont want to wake up to my fishes dead.. I only have one tank.. or leave it and see what happens im stressing literally sitting in front of my tank and looking at them. .. all the fishes are fine except one of them it keeps chasing and grabbing the other fishes tails. (Aggressive)😭

EDIT its killed 2 fishes ... I took him out probably try to see if the fish store can have him