Kurt is correct. Other than clown fish, and even they have to be careful initially, a carpet anemone will eat any fish, shrimp or crab that contacts it.
The plus is clown fish will readily adopt a carpet anemone, as for many species, its their natural host.
The minuses are several, not the least of which they get big; very big, as in two feet in diameter plus for Stichodactyla haddoni, and up to four feet across for S. mertensii. S. gigantea is a mere 16 or 17 inches across, and thus the best species for a tank the size of the poster's 55 gallon.
They MUST have perfect natural sea water levels, as they will not tolerate a whiff of Nitrate. They will respond by shriveling up in the presence of a few PPM
of Nitrate, and will quickly die unless the situation is rectified.
They also need bright, reef-like light. You can keep a carpet anemone under VHO
tubes, but a halide or two wouldn't go amiss.
Without the proper lighting a carpet anemone will shrivel and die.
They also will wander until they find a spot they like, so if you keep corals with your anemone, it would be best to allow the anemone to establish itself first, before adding corals, as make no mistake, carpet anemones can really sting, and can easily kill a coral.
Each species has specific substrate needs. Haddoni needs a deep sand bed, so it can retreat if threatened, like by the hand of a fishkeeper. Gigantea attaches to hard surfaces, like a large piece of live rock that's suitably even. Mertensii likes to put its base in a crevice or hole in live rock, and spread itself over the surrounding rock.
All of them survive better in the presence of clownfish.
Carpet anemones should be fed once or twice a week. They can survive on their symbiotic algae, but experiment with different meaty sea food to see what it'll take. Needless to say, the tank must be kept scrupulously clean, so any uneaten food or offal from the anemone itself should be PROMPTLY removed.
Carpet anemones can be successfully kept, but are one of the most challenging animals to care for, even more so than the common anemones in the hobby. But they are fascinating, strangely beautiful creatures.