I had a nasty entourage of aptaisa when I started out. Must have had atleast 50 of them in my 24g.
The Kalkwasser route works great. So does Joe's Juice, a recent addition to the aptasia control products that are currently on the market. Just syringe a drop above their mouths and they're dead in seconds. I've used both with great success.
Some even claim vinegar or boiling water injected into the base works too (though I have no experience with this route myself).
If you go for such methods, be sure to turn off your current for a bit (powerheads, skimmer, etc..). When aiptasia die they release spores. Sort of like an emergency 'if I can't live, I'll pass on my seed and make 5 more of me' sort of thing. With your current off, you'll minimize this.
Joe's Juice comes with a syringe. If you need to get one elsewhere, I'm sure a pharmacy store will carry all sorts of sizes.