Like has een said, get two more girls. :) They'll add more life aswell as spread the aggression. Make sure they're about the same size as the ones you have already.
Also, if the new additions don't help you can always give lilac her own tank. you can get a 2.5gallon critter keeper pretty cheap, or alarger tank if you wanna spend the money on a 'real' one.
-duckweed- -black and white marble, anything but VT-
-javamoss- -koi marble, anything but VT)
-hatchable brine shrimp eggs-
-indian almond leaves-
-Melano Halfmoon male-
-steel blue or royal blue CT or HM female-