As you can see from Liz's 5g nano-reef aquarium picture, her beautiful live rock formation is nicely populated with a variety of blue, red, and green soft mushrooms, along with a few other types of corals and a Feather Duster (tube worm).
Smaller tanks make great Do It Yourself (DIY) Projects. They are fun, easy, inexpensive and go together quickly. Just modify the DIY 55g Glass Aquarium Plans to meet your needs. There are a number of Canister and Power Filters and Mini/Nano Reef Tank Lights which will allow you to keep corals in your new tank.