Clowns do not need an anemone to live a healthy and happy life in captivity. You can take that statement as a fact.
All anemones will have horible survival rates, and few live over 4 months, esspecially the Ritteri Anemones. An Anemone of any kind should not be bought for the clowns. Anemones may move where they wish, which gives them a high chance of stinging other corals, getting stuck in a powerhead, or getting stung. Anemones are actually not "Real" reef inhabitants. Many have only been recently introduced into reef aquariums. For the most part, anemones do not survive well in the home aquariums. Even if you had the perfect setup, it probably will still die.
If you want something for your clowns to host in, I would recommend a Toadstool leather (Sarcophyton sp.) for your clownfish. These will make perfect hosts for the clownfish, and often will resemble an Anemone. Many other types of leathers including the Devils Hand (Lobophytum sp.), Spaghetti Leather (Sinularia flexibis) and Finger Leather (Sinularia sp.) will also work fine.
Other good hosts include the Colt coral (Cladiella sp., or, Alcyonium sp.), Kenya Tree (Capnella sp.-should be added to a mature aquarium), Alveopora (Alveopora sp.), Torch Coral (Euphyllia glabrescens), Hammar Coral (Euphyllia ancora), Bubble Coral (Plerogyra sp.), Green Star Polyps (Pachyclavularia sp.), Xenia (Xenia sp.?), Frogspawn (Euphyllia sp.), etc.
There were many I never meantioned, only due to the care level required for them, and the difficulty of keeping them sucessfully alive. Some of these include the Flowerpot (Goniopora sp.), Plate Coral (Heliofungia Actiniformis), and Elegance (Catalaphyllia sp.) to name a few.
Please do not buy an anemone. Save your money and buy it on something that will actually live.
Anemones do best in the ocean, which is where they belong.
Take Care,
Graham