I have a pink condy and a Tomato Clown in a 10 gallon in my living room. I had to seperate them from my Red Sea Max because they were killing everything in my tank ( I litteraly watched them do it). I planed on keeping them in there until I could get a bigger tank, but they settled in nicely and have been fine together for the last year. I have a 3 or 4 pound piece of live rock, two 10gallon filters, 50/50 Actinic/10K lighting, heater, Maxi_jet 400 flow pump, 2 blue leg hermit crabs, and a few shells and pieces of limestone with fossils in them that my wife found for decoration.
I do a water change on this tank once every 3 weeks. It is also FULL!!!! of Amphipods and Sycon sponges. The sponges seem to come and go, but all my levels stay the same, so I am not sure what that is all about. So, YES, you can keep an anemone in a 20G. At least a pink condy anyway. One thing I did notice is that the anemone does not get as big as he did in the Max. He would expand to about 5" or 6" and will only get to about 3" to 4". I hope that helps and good luck.