I have very good luck with a small syringe (bd or monoject, used for injecting meds like insulin). I fill it with lemon juice, insert the syringe directly into the mouth of the aptasia (or in my case, majano) and give a small squirt of lemon juice. Even if it closes, I just push the syringe into the body and sqirt.
Works everytime, doesn't disturb the neighbors.
db