What are you using for lights over this tank??? When an anemone climbs to the top of a tank it usually means he is trying to get closer to the light.This tell us that your lighting is FAR too weak to have this anemone in that tank...Also(as stated) a 20 gallon tank is WAY TOO SMALL for a carpet anemone.If he does live,in a year he will be over 12" in diameter.Mine is about two years old(maybe more) and easily measures 20"across....