I would think it has something to do with the health of the anemone, the healthier the nem, the more powerful the zap. I had a tube anemone that ate a few shrimp, a couple of lawnmower blenny's, but the last straw for me was when it stung my Copper Butterfly fish to death... I found the fish locked in its tentacles. I rehomed the anemone, and vowed to never put another one in my tank. My clownfish hosted the intake tubes of the filter, they were happy enough without me putting a beautiful fish killer in there with them to wallow on.