I have a 25g african cichlid tank with 6 fish inside(1 female red zebra, 1 venustus, 2 aulonocara, 1 female tropheus, 1 male zebra) for 1 month now. Yesterday I introduced one more fish (aulonocara albino) and within 2 hours the other fish chased it to death (especially my red zebra). All the fish are about the same size (4-5 cm) except my venustus which is 8 cm. Did I do sth wrong? It's like they will not accept any other fish in the tank... Must I do sth before introducing any other fish???