theres lots of ways: theres store bought products like joes juice, or you can mix up a thick paste of kalk and tank water, some people use boiling water or lemon juice. any of these needs to be directly applied to the mouth of the pest anemone. easiest way is to use a syringe, joes juice comes with one. the other way is if you can easily take the rock out of the tank you can use a small blow torch to burn the aptasia right out of the rock. these things multiply like crazy and can sting your other corals so get rid of it now.