It was suggested to me to add Damselfish so I have. 2 Yellows which seem to be doing well. Watch out for intraspecies aggression and they will do best with a steady specific gravity and low ammonia. Introduce them carefully (no light stress, drip acclimation, and regular feeding). It's been almost a week and I have had a positive experience -- so far. $3.00 a piece at the fish store.
Perhaps another choice would be Green Chromis -- I've lost 2 of 3 due to improper care. Be careful. $3.00 a piece at the fish store.
After this, you could add Long-Nosed Butterflyfish which are a good second choice -- hardy and eat well and will calm down the aggressive damselfish.
Take your time -- this is only 1 idea.