I feed mine once per 3 days. I alternate fronzen seafood that you buy at the grocery store. Some of the things I have fed them is clam, squid, fish, shrimp.
I chop the meat up very finely and mix it with 1ml of phytoplankton in a needleless syringe. I then spot feed the aenmone by squirting it directly into its tentacles. If all of the meat does not come out on the 1st squirt, just suck some tank water into the syringe and try again.
It is safe to feed your anemones flake food when you are lazy/don't have fresh seafood, but don't do this more than a couple times a month.
One last word for anemone success: UNDERFEED!