here is some info on them from the supplier
General recommendations are 1 per 10 gallons of aquarium volume, it depends a lot on how many aiptasia you have. If you only order a few it will be the next generation of berghia that do most of the work, so it can take several months to completely eradicate the aiptasia problem. I've had many that have wanted to take care of the problem and ordered a couple extra adults and the aiptasia have been gone in just a few weeks
1. Berghia only eat aiptasia nothing else
2. Berghia are not toxic they will not hurt your fish or your tank if they die
3. When you run out of aiptasia berghia will roam a lot more and you can often catch them out and resell them to other reefers.
4. There is a good chance you will not see them again for quite some time once they are added to a reef.
5. They can handle high flow once acclimated and settled, i've had them in my 20,000 gph SPS tank
6. Fish and peppermint shrimp can and will eat them, they stay within the rockwork and hunt at night to avoid predation. The primary danger is just when you initially put them in the tank.
shipping is ten bucks per person right now