After a very long time, a tired Mary shuffled over to the vials. In her tiny form, she had to hoist herself up onto the soapdish and grab the Growth vial with both hands. She drank about a quarter of it, slowly of course due to her smaller mouth and throat, when she started to feel its affects. She quickly slipped back into the water, but as she did her scales brushed against the Mermaid vial, knocking that into the tub as well. It fell to the bottom and cracked--not enough to break apart, but enough to cause the potion inside to seep into the water.
Mary bobbed on the surface, grasping the vial in one hand. She had grown back to a foot in height. That's when she looked down and saw another vial on the bottom of the tub. She put the growth vial back and dove, grabbing what she discovered was the Mermaid vial and bringing it to the surface.
No longer under the water pressure, the crack in the vial let out even more potion, now over half empty. Mary quickly cupped a hand under it to catch the leak and tossed the vial up and into the sink. She looked around, disappointed in the loss of the vial. Now this may be the only chance as a mermaid she'll ever get.
She waded around the tub in a sour mood, not wanting to return to being human with no hope of swimming as a mermaid again. Then she felt something. It was a tingling, like she had felt before when her legs had grown scales, only now it was in her chest.
Sure enough, Mary saw blue-green scales spreading up her stomach from her mermaid tail. They invaded her chest, covering her breasts in their shiny folds. Her nipples disappeared underneath them. Her arms followed suit, the scales surrounding her elbow fins spreading to dominate everything from her shoulders to her fingertips. The webbing on her hands grew to the full extent of her fingers, now a translucent green. Talons extended from her fingertips as well.
Her fish tail developed another, smaller dorsal fin halfway between her original one and her tail. Meanwhile, scales had covered her entire face. Her teeth shrank into her gums and her eyes enlarged, becoming round yellow orbs. Her lovely hair shortened, completely receding into her scaly scalp as the fin on her head spread out and divided into three, one on each side of the original.
The fins on her arms quadrupled in size, and her tail fin doubled. Mary lashed about in the water, confused at her transformation. Her mind seemed unfamiliar with her surroundings. Where was the sky? Where were the plankton, the seaweed and all the other fish? In the back of her mind, she could faintly recall her name and original species, but as her mer-fish brain reshaped itself the idea of being human seemed but a fleeting dream.