I have 2 serpae tetras and 1 longfin blackskirt tetra in a 10 gallon. One of the serpaes keeps chasing the blackskirt around, to the point where the blackskirt is beginning to look stressed. (Nitrites are also high so that is stressing him as well, I do water changes to keem them down.) But with the serpaes chasing him the blackskirt just goes to the back right corner of the tank and sits near the filter intake. Is there anything I can do to stop this? Maybe he needs another of his kind? Please help, I don't want to lose a fish.