I bought 4 damsels 5 months ago. One of them has grown and the other 3 haven't any ideas why?

Question

I have a 45 gal saltwater tank.

Answer 1

Damsels are territorial, so the dominant one in your group has probably stakes out the best place for itself (right where you put food into the tank) and chases the others so they aren't able to get as much food.

Also, if they aren't all the same species, some may naturally larger than the others and have a proportionally greater growth rate - this makes them appear to be growing faster because they have more "growing" to do.