I am convinced Aiptasia come from nowhere.
However, to qualify that statement. In my case they always show up on the same rocks. You can wipe them out on a rock, but sometimes months later on the same rock they are back.
For instance; Yesterday I treated a rock recently purchased at a LFS for one Aiptasia. I looked at the rock today, and found five little Aiptasia. So today I treated that rock again with boiling water.
I am willing to bet my last nickel that in a few days there will be more.
