Aiptasia (Aiptasia pallida and friends) is a translucent brown anemone with long stinging tentacles and a tall, slender body. Majano (Anemonia spp.) is shorter and stockier with shorter, more bulbous tentacles, often with green or brown coloration. Both arrive on live rock, frag plugs, or hitchhiking on incoming coral. Both reproduce by pedal laceration - even a tiny piece of foot tissue left behind after removal grows into a new anemone within weeks.
Cutting an Aiptasia or majano in half does not kill it. It produces two anemones. Pulling one off live rock leaves enough pedal tissue behind that you have just multiplied the population. Every reefer learns this once. The methods below are reliable because they target the entire animal, including the tissue still attached to the rock.
Berghia stephanieae is a tiny aeolid nudibranch that eats nothing but Aiptasia. Add 6-12 to a tank with a meaningful Aiptasia population, leave the lights on a normal cycle, and within 3-6 months the population is gone. The Berghia themselves do not survive without their food source, so they fade out naturally once the work is done. Berghia do not eat majano - if you have both, you need a different approach for the majano. Berghia will not survive in tanks with peppermint shrimp, mandarin gobies, butterflies, six-line wrasses, or anything that eats small invertebrates.
Real Lysmata wurdemanni eats Aiptasia. The closely related Lysmata boggessi often sold under the same name does not. Buy from a vendor that explicitly identifies the species. Add 3-5 peppermint shrimp to your tank, feed them lightly so they go after Aiptasia for food, and they will reduce the population over a few weeks. Peppermints can also nibble at zoanthids and clam mantles, so they are not perfect roommates for high-end zoa collections.
For larger or isolated specimens, direct injection of a paste-thick kalkwasser slurry, F-Aiptasia, Aiptasia-X, or boiling lemon juice (yes, that works) kills individual anemones cleanly. Use a syringe with a blunt-tip applicator, inject directly into the oral disc, and pull back so the anemone closes around the substance. Dead Aiptasia turn white within minutes and detach from the rock within a day. This method scales poorly past 20-30 anemones - at that point you are better off introducing biological control.
The Aiptasia-eating filefish, also known as the matted filefish or bristletail filefish, eats Aiptasia and majano with enthusiasm. They are reef-safe most of the time but occasionally develop a taste for Acanthastrea or zoanthid colonies. Best for FOWLR or simple soft-coral tanks; risky for high-end SPS or Acan systems. Add one fish per 50-100 gallons; pairs are aggressive toward each other.