[philter4]

It will grow back on a couple of conditions, if it is a birth defect or there is damage to the body at the base where the fin is that would cause permanent damage to the cells that regrow the fin, it won't grow back. Did you notice if the fin was there when you perchased it? One more thing fire fish usually don't like each other and if they are not a pair one can stress the other one out to the point that it doesn't do well. In the wild they are very territorial and live either single or in mated pairs. I don't think they would tear each other up though so the dominant one probably didn't chew the fin off.