When they are eating well and getting fat, then you can start to limit the amount of food you feed them. I feed 1 cube per day until they are fat, then decrease to 1 cub in the afternoon only.
Another trick is to disturb the aiptasia. The tentacles are where the sting cells of the aitptasia are, once I see they occasionally pick at the aiptasia, I turn the rock that contain the aiptasia upside down or side way. This will cause contraction of the tentacles, and expose the soft, tasty, non-stinging part of the aiptasia open for attack by the CBB. This often cause them to destroy the exposed aiptasia. Once they are well on their way to eating aiptasia, no help is needed