Many times people try peppermint shrimp for aiptasia eradication. Many times it does not work. We recommend removing peppermint shrimp prior to introducing Berghia to the tank. Here's a cheap method for catching peppermint shrimp that I made in a few minutes. (It also caught two emerald crabs and a yellow clown goby.)
Use a 32 oz. water bottle or 2 liter soda bottle depending upon how much room you have available in your tank. Cut off the top third part of the bottle and invert it back into the bottle. You can hot glue or Superglue gel it in place if it does not stay snugly on it's own.
Fill the bottle trap with tank water and remove most of the air. You may need to anchor the bottle to the bottom of the tank like I did using a small rock. Try to place the bottle as close to where the pepps are hanging out.
I bait the trap using a piece of silverside fish. Use a heavier piece of food like the fish or freeze dried krill. (Brine shrimp or mysis shrimp tend to float out of the bottle)
Place the trap in the tank after your lights go out. Because fish might go in the trap we do not recommend leaving the trap in overnight. A couple of hours is usually all it takes to catch the shrimp. If it does not work the first night, try again the next.
Here's one a friend made.
Other ways of catching Fish and salty critters
If you fear you have something in your tank that may eat your berghia, check out the following link for great ideas on how to catch fish and critters in your display written by Anthony Calfo and company.
How to Catch Fish and Critters in Displays (Reef Central)