The best natural defense against Aiptasia is Berghia (Berghia Nudibranchs), as their sole diet is Aiptasia anemones. Berghia are non-toxic to corals and fish and are considered to be completely reef safe.
Reeftown recommends at least 1 Berghia per 10 gallons or 2 Berghia per 10 Aiptasia plus one or more per additional 10 Aiptasia for a small infestation, but NEVER less than 2 pair of Berghia. Berghia need each other to Hunt, Feed & Breed. If you're not sure, it's best to overestimate rather than underestimate. Read more about selecting the right number of Berghia for your aquarium on our Estimating Needs page.
Reeftown recommends the Medium Berghia (Click Here) that lay eggs frequently and are available in greater quantities.