Have you had Twizzler long? If she's pretty new or this is only her first or second thunderstorm, she may not be used to it. Plus, as a fish, she can probably pick up on the changes in the humidity, the sonic pressure, the electricity, all that scientific technobabble. Anyhoo, I don't see how this could possibly hurt Twizzler unless she does something drastic like constantly get so spooked she swims into the walls or jumps and hits the tank cover. And if she's that spooked, it might be time to hit the storm cellar.