Cirrhilabrus

[Gouldian]

my six-line wrasse terrorises any other wrasses to death. =/

Six line belong to the Pseudocheilinus species, they are the more aggreesive lot. Herewith is a write-up in Reeking Magazine giving a better idea of their habits.

"However, it is also not uncommon for these diminutive fish to become rather boisterous to the extent of becoming hostile. Once established in the aquarium, mixing additional fish, or even shrimp, can be an exercise of futility. Smaller fish may be consumed, while fish of equal size or slightly larger are likely to be harassed. Sometimes the harassment will subside after a few days, other times it will continue until the death of the targeted animal. In either case, it is best to take steps to avoid this confrontation. Some tips to minimize these problems would include adding the Lined Wrasse last, separating individuals with egg crate material until aggression has subsided, and designing the aquarium's aquascape to contain a lot of hiding places. In some cases the removal of one of the fish may be required to restore equilibrium. Maintaining individuals of two or more Pseudocheilinus species in the same aquarium is not recommended."

Extracted from: http://www.reefkeeping.com/issues/2003-08/hcs3

That's why, I personally like the Cirrhilabrus.