How long has your tank been running. A well established tank is not necessarily a needed factor (nem's have been successful kept in tank less than 1 yr old) but it is a good thing to have on your side. The only reason why they say "established tank" is because for a anemone your water parameters need to be stable. They don't like change. Some nems like higher light some like medium light. It will find where it likes. I do agree with
@AcroNem about feeding them once a week or so. Another thing to add is they like flow. Not too much to blow them out of their own flesh but enough to have them wave in the current. With that being said please. COVER ALL POWERHEADS WITH A SPONGE FILTER! I cannot stress that enough. People don't do that and then they wake up one morning to a cloudy tank without an anemone.