my experience is slightly different than the advice you've gotten so far. i kept one in a biocube 29 for a few months before i cracked the tank and had to move more quickly to my elos 70. but it was doing fine in the biocube with regular water changes. note my biocube had been modified with the addition of an ~10 gallon sump. that said, it was a relatively small gigantea (maybe 7 inches extended).
so if your anemone is smallish and you dont have other stuff in the tank taking up room (like lots of sps / lps / rocks constricting it's ability to get good flow and stretch out), i think you'd be fine for quite a while. if you dont feed it directly it wont grow quickly.
my 2 cents.