Joes juice will kill zoas but a lot has to land on it and it has to stay there for a while.
I purposely killed some zoas with JJ and I had to cover them twice. A thick blanket of the stuff, it looked like snow. I did it on purpose and it didn't kill all of them.
If you wanna get just the aiptasia, put some jj in a syringe, then put a tiny piece of meat in the end of the syringe. When the aiptasia "opens up" to accept the meat, shoot a tiny bit of JJ in it's mouth and he will die soon. If it gets on your zoas, just blow it off with a turkey baster.