How does Tova hurt herself in "The Technically True Story"?

That evening when Tova comes into work, she pulls out an old step stool next to Marcellus's tank and carefully climbs up the stool. She slides the cover off the back rim of Marcellus's tank. She trails her fingers in the water. He rises to the surface and winds his tentacles around her wrists and forearm. She talks to him. She starts to clean the rim of the tank. Part of it is just out of reach. She shifts her weight and stretches her arm. The stool wobbles. She falls and hurts her left ankle.