I was just facing a similar problem in my 20 gallon. We had a red firefish that got along with our percula really well, but one day she decided to find the only small opening in the covering of our tank and hop out. Saddened, we decided to get another one, and found a really good looking big guy from the LFS. When he got in the tank, he was great, swam around, explored, etc. Then the percula turned into a bully, and the firefish went into hiding. The firefish was so afraid of the perc that I had to target feed it in a little corner the firefish had made its own (right by its dart hole). We recently added a green chromis to our tank, (this is about a month after buying the firefish), and low and behold, the firefish came out. Now all the fish swim together, and seems like they all like to pick on each other. Except for our green clown, he just hides under our mushrooms and watches everyone act silly. I don't know if the chromis made a difference, but Dr. F &S catalog says that they help bring shy fish out of hiding...