Hi I have an nannacara anomala male in a 15 gallon and he has gone all colonel kurtz on me. every time i try to add a tankmate he tries to kill them. so far hes killed a filter shrimp, three golden barbs and a betta female. I guess that black phantom tetra could deal with him but i already have them. Is there any tank mates that you would recommend or should i keep colonel kurtz on his own?