They may do fine for a short period, however the purple will not not tolerate the Kole tang in a small environment for several weeks. I have an Achille's in a 40 gallon breeder and had a large Angel with it. Needless to say I had to remove the angel due to aggression. Achille's aggression=Purple Tang aggression in a small tank. Pick up another small tank, most are pretty cheap. A 20 gallon long (Aquaeon/All-Glass) at Petco runs about $25-$30.