I voted "other." I can't say if it is avoidable or not. By rookie mistake (wet rock from LFS) I started out with an infestation before I stocked my first on-purpose critter. However, I bought a real (Lysmata boggessi) Peppermint Shrimp, and that rascal went to work right away. It was amazing to watch him hammer away at an Aiptasia, absolutely destroying it! I sort of felt bad for the Aiptasia! My tank was Aiptasia-free in about a week. YMMV. I understand that these shrimp can lose their taste for the unwanted nem once they get a hold of LRS Reef Frenzy or Mysis. It is possible that yours could have had lost its appetite for them before you even bought it. I may have been lucky with mine, but it demonstrates that avoidable or not, Aiptasia are not a foregone conclusion in a tank.