10 Gallons
A 10 gallon SW is possible but not recommended. I was able to get a 10 gallon SW system to live quite well for a year before I finally transfered the fish to a larger tank. The main problem is that it takes a lot of aeration in such a small aquarium. This causes a lot of water to build up outside the tank, which can cause electricution problems later on (salt is an excellent conductor of electricity). As far as fish that could survive in a 10 gallon tank, I'd start with converting some molly's into SW first. This would be because it will probably be 3 to 4 months of cycling the tank before even a damsel will survive well in such a small tank. You will also want a lot of live rock once you do get actual SW fish. This is because SW fish are very territorial and are overly stressed when too close to other fish. They will need plenty of places to hide to feel secure and to keep them from attacking each other. As far as what fish could survive, I had a damsel,a clown, a bunch of live rock,and some snails and hermit crabs. IT worked and was healthy but I still wouldn't recomment it. IT takes tons of maintainance and does not necessarily provide an ideal living space for your fish.