From what I can find from different websites on the internet says that Sodium Hydroxide will kill aiptasia when it touches it or a few seconds later. It will also kill algae and corals. If you have a big algae problem, and you have to remove the algae today for some reason, you can use Sodium Hydroxide to kill the algae. You can also use Sodium Hydroxide to keep zoanthids or some other coral that spreads very quickly under control.
You have to be careful when using Sodium Hydroxide around corals that you dont want to kill.
My favorite part about using Sodium Hydroxide to kill aiptasia is: you don't have to inject the aiptasia to kill it. Just place a drop(or more for bigger aiptasia) in the center of it.