I usually fed my RBTA once a week a whole silversides, but since I ran out, I may stop feeding it that because I have read a few things on what they can do to anemones if they are not good quality, so I am switching over to using shrimp...and maybe some salmon or other kind of high quality raw fish meat.
However, if you want an anemone to grow, I would say twice a week, and vary the diet a little.