You will be amazed by the great diversity of vertebrates and invertebrates that you can keep in your small piece of the ocean. What you can keep all depends upon your nano reef equipment. Some corals will require higher light than others, and some fish will need a larger aquarium than others. All of the critters listed here are nano reef safe.
Nano Reef Fish
Fish are a great addition to a nano reef, but you must take extra care of you nano. You will have to be sure to keep up with your water changes, because a small body of water can collect nitrates quickly. The following fish are nano reef safe: true and false percula, banggai cardinals, pajama cardinals, royal grammas, fire fish, clown gobies, pseudochromis, basslets, and most damsels. While you are not limited to just those fish, these are hardy and good for beginners. You may find some people keeping fish that grow larger, such as dwarf angels or tangs, in their nano reefs. Often times they either do not know any better, or they intend to move the fish into larger tanks. I strongly urge you not to try this. It causes more stress on the fish, and possible problems for the future.
A good cleanup crew will keep your nano reef running smooth, and free from algae, detritus, and other unwanted wastes. Commonly kept cleanup crew critters are red leg hermits, left-handed hermits, emerald crabs, sally light foots, turbo snails, and astrea snails. Keep one snail per 1-2 gallons, and one crab per 3-5 gallons. Do not keep more than one of the larger crabs in a nano reef (sally light foots, emerald crabs, etc.)
Nano Reef Shrimp
Shrimp are a great addition to any nano reef. One of the best suited for the job is a cleaner shrimp. There are two different types of cleaner shrimp available to the hobby; the skunk cleaner shrimp, and a fire shrimp (aka blood shrimp). Not only will a shrimp provide extra help to the cleanup crew in keeping your sandbed and rocks clean, it will also rid your fish of parasites. If a fish in the tank gets ich, a sickness in which white parasites attach to the side of the fish, a cleaner shrimp will work to eat the parasites off of the fish. Other options for shrimp include peppermint shrimp, and camel shrimp.
Nano Reef Corals
The sky is the limit when it comes to what corals you can keep in a nanoreef. There are just a few guidelines to follow when you choose a new coral. You will need to remember that some corals need more light than others, so be sure to take your lighting into consideration. Another thing to consider is whether or not the coral has sweeper tentacles. In a small aquarium, if one coral puts out it's sweeper tentacles in the night, it will be able to sting almost every coral in the aquarium. An example of a coral that does this is the elegance coral. Be sure to research all of your corals before you purchase them to be sure they will make a good addition to your nano reef. Also, don't be tempted to fill your nano reef up with corals. You should provide enough room for the coral to healthily grow, before having to frag it back to a smaller size.