IMO it depends on the species. While some of the less potent anemones do fine with whatever they catch out of the water column (i.e., BTAs, mags, etc.), I beleive the true "killers" (i.e., carpets) need to be fed occasionally for optimal health. IME, when I go more than 2-3 weeks without feeding my gigs and haddonis, they start to shrink a bit and overall look less "happy." They perk right back up the day after feeding and always look their best for 2-3 days post-feeding. They have such sticky tentacles for a reason!