I agree with eatbreakfast. The firefish looks scared. As mentioned a clownfish can be a bully in a small space. There may have been an early display of aggression and you never saw it. Or it may have been in an altercation in the store/wild. Give the firefish a good safe place to hide, to reduce stress. They like sandy caves under rocks. Another option would be an acclimation box that goes inside the DT.




