Good suggestion Titan. Is that a clown angler? They only get to like 4", so perfect for a small tank. I think an angler is a great newbie fish, if it's the only fish in the tank. The reason people normally have problems is when putting them in a community tank. Anglers have serious "eyes bigger than their stomache" syndrome. So many times they will choke and die while trying to swallow a 7" tang.
The problem is that you shouldn't be feeding freshwater feeder fish to saltwater fish. It is bad for the saltwater fish, unless you know where to get saltwater feeder fish, like small silversides. If you get one, you can feed them live feeder shrimp instead. Shrimp are fine for his system and it's still a lot of fun to watch an angler pounce on a shrimp. :happyfish
A fu manchu lionfish might work if you get a smaller one. They eat the same as the angler. You are really pushing it with a 10 gallon though, a 30 gallon is the normal minimum size for an adult. They get to around 4" also.