the more important aspect to keep in mind as far as stocking densities is the mass of the fish, not so much just the length. One 10 inch lionfish will produce more waste than 10-1 inch damsels. Keep in mind the amount of beneficial bacteria, if it is a freshly set-up system, keep a close eye on the ammonia, in a high pH it becomes more toxic, so do water accordingly. There are some really great books out there aimed towards the beginning hobbiest. Happy Fishing