Small amounts of HCl is not bad for your tank Truck. I've tried injecting concentrated HCl (10Molar) directly into the aiptasia directly with a needle/syringe to no effect. Also tried using 10M Sulfuric acid (stronger still) with no effect. Saturated Kalkwasser (joes juice) has limited success but really only kills 20-30% of the time. Havent tried concentrated ammonium hydroxide yet (stronger base than kalk) as I'm waiting to see if my Berghia survive and do any good.
If you ask me, the only options for dealing with them are Peppermint Shrimp, Berghia Nudibranchs, or if you want a man-made one, burn them into little piles of goo with a torch. Johnny's suggestion about using putty or superglue can work too, but I've also had the little punks wiggle out from under those barrierd...