I agree with most of the comments. The xenia, although pretty cool, do split and spread. the kenya tree breaks off and forms new colonies as well. I constantly have to sell back to my LFS xenia. Hairy mushrooms are great for the 12g nano. Hammer, frogspawn are great too (they can sting other corals though). I also have some torch or candy-cane coral in mine; those things grow pretty quickly in my tank. I fragged my torch coral and now I have two stalks with 5 torch's each. A firefish and a small goby add a nice touch to my nano tank also. Although, my first goby was a tiny yellow goby and, believe it or not, my hairy mushroom ate it. Now I have a citron goby in there and he is fine. I change two gallons out each saturday and my tank parameters are great. I feed a very small amount of mysis each night to the fish and sometimes to the coral also. It is very easy to maintain.
But, as everyone else stated, you must do research. I've been doing saltwater for 8 or 9 years and I still read a lot and learn a lot....especially from this site. Some folks here dont have more than a year of experience but they are still very knowledgeable about saltwater aquaria. We all learn from each others posts.