I have used vinegar (both distilled and balsamic) very effectively. I have also used lemon juice with good results. You need to inject the stem with a small amount to ensure a kill. I found that engulfing the water around the head will only stun the aptasia. You need to stick it, that's why the small needle. The tiny amount of acid that I used did not effect the pH of my 26 gal tank at all; don't worry about a pH change.
At first I tried a syringe with large hypodermic needle and squirted the hole it retracted into, but had better luck with a small needle. EvilE had a good suggestion--using a insulin syringe, but purchasing one may be a problem since they are sold in large amounts. Several Pharmacsists did not want to spend time with me for a $1.00 syringe. It might be very easy for you to ask a diabetic friend if they have an extra. I bought my syringes at an old fashioned, tack and feed store for $0.78 each. You may want to try a veterinarian (per EvilE suggestion).
About a year ago I had good luck with a Peppermint Schrimp eating the Aptasia. But after the Aptasia were gone I thought the shrimp were starving so I returned the shrimp to the LFS. About 8 months later I discovered new Aptasia and bought two new Peppermint shrimp---I haven't seen them, but I have seen the Aptasia. I think that some shrimp will help you and others won't.
In my opinion, now is the time to rid your tank. They are like Dandylions with a stinger; pretty to look at but.....
Good luck.
