WARNING: Do not get a berghia for 10 or so aiptasia. The only thing that they eat are aiptasia. It will not take any substitute to aip and when the 10 are gone, it will starve. The animal will not live without constant regeneration or repopulation of the aiptasia. If you get one and it eats it, you need to get it into another tank with aiptasia ASAP!! There are much easier, less expensive, and hardier solutions to the aip problem. The berghia is an easy way out, but it is at the expense of other animals. And yes, when they die, they are toxic to the tank. Granted, one dying will probably not kill of your stuff, but it does not make it a pleasant living environment.
First option: Joe's Juice. This stuff kills aiptasia through the aip trying to eat it(it tastes good to them) and getting a nice strong dose of Kalk along with the food. They shrivle up and die
Second option(can be used along with the first to get even better results: Peppermint shrimp. These guys are known feeders on aiptasia, but the good thing about these guys, they can eat algae, pods, flake, pellet, or frozen food also. They are not a cure-all like the berghia, but they are much more resilient, reliable, and hardy.
Trust me, you do not want to bring a berghia upon yourself unless you are willing to have a tank dedicated to aiptasia propagation.