I've got 3 freshwater tanks with knowledge in that category with limited knowledge in the area of marine fish compatibility. My girlfriend has a 13g nanocube with a blue damsel and some BL hermit crabs and no corals, just live sand and live rock. She wants another fish that can live with the damsel that wont kill each other. We know thats a bit small for a damsel to be in, but is there something? She doesn't want to take the damsel into the LFS quite yet. Is there even an option at all? lol, I do have my doubts on anything being capable.