That depends. 20 gallons are much harder to keep stable than larger tanks. The amount of success you have with it depends on your experience. Are you experienced in saltwater? If so then you shouldn't have much trouble, otherwise you might want to start larger. My first saltwater tank was a 20 but that was after lots of research and a significant amount of experience with difficult freshwater fish. My saltwater operation has now expanded to include 2 20s, a 55, and a 90. All are reefs except one of the 20s. So- it can be done fairly easily provided you do some research and pay a lot of attention to it. I'd suggest small damsels- one, maybe 2. I also breed clownfish and have heard of people succesfully breeding pairs of ocellaris or percula clowns in a 20 with no fish tankmates ofcourse. The desire for many different fish is easily curbed in saltwater by adding crabs, shrimp, urchins, and after a little experience; stars, mushroom anemones, and zoanthids can all be maintained in a 20 with relative ease and can be quite breathtaking. There are even more options available to those who truely know what theyre doing.
