Berghia nudibranches only eat aiptasia or you can make a solution of kalk and hot vinegar and inject them. If you only do a small amount at a time you shouldn't get a swing in ph. I personally don't like the lemon juice method as there is potential to add other contaminants such as fertilizer, insecticides and/or preservative agents.
Also little FYI if anyone buys a peppermint shrimp and it doesn't eat even the smallest aiptasia you will then know that you actually bought a camelback shrimp. They are identical in appearance and it is a common occurance that they are sold as peppermint shrimp. It's one of the perils of relying on livestock to take care of aiptasia.