Freshwater dip more likely to stun than kill, and if they are in a rock then you'd really need to leave it soaking or just dry out rock (depending size could be days or longer).
Don't recommend adding rock to your berghia tank.
Keep my breeding tank bare. ... though I do have a Pod Hotel in each so they can hide since my room is also my office and during the day can get bright.
Instead, if you are trying to get the aiptasia from the rock - put that rock in a bucket (and depending how long you keep that bucket going, get it a heater as aiptasia like 77-80 for optimum aiptasia breeding (berghia are more flexible about temps).
Last night from PetSmart I just picked up a Fluval 3-10gallon 10w heater to start another bucket of aiptasia growing