My aptasia infestation was solved by the addition of a Racoon Butterflyfish.
This is not a reef safe fish, but in my 55 gal FOWLR
tank, it is great. It ate all of the aptasia, big and small, within 3 days. The minute the racoon was loose in my tank, it began attacking the aptasia and did not stop until they were all gone. This fish is beautiful and friendly, and now eats Mysis shrimp and flake food heartily.
The Butterflyfish enjoyed eating aptasia so much that I am now growing some in another tank for it!