Your legs begin to feel weak as soon as the water hits you, but you also feel more aroused. You slump against the tile. The mermaid you saw in the carnival mirror still dominates your thoughts. You slowly slide down the tile into the tub as your legs continue to lose strength. They are soon stuck together and orange scales begin to appear all over them. Your feet flatten. Your legs spasm and flail as they slowly merge into a tail. You realize you're turning into a merman.
The thought of turning off the water and trying to get out of the tub doesn't even cross your mind as you're enjoying yourself too much.
Then you remember the merman in the carnival mirror had a dark blue tail; the mermaid that had an orange tail. You feel something against your shoulders and reach up to find your hair is growing longer. Your chest feels warm; unwilling to look down at it and you hand moves to inspect instead. You find yourself cupping your growing breast. It's so sensitive. Soon it has swelled to what you guess is a DD, like the mermaid in the mirror.
Curious, you push your breast up toward you face and find you can reach the nipple. You give it a gentle bite with your teeth. That does it. Your back archives involuntarily and you climax, splatter semen on yourself and the tile walls. It's the biggest, longest climax of your life. Then your dick grows limp and begins to slowly retreat into you body. At this point you're not surprised.
The water from the shower continues to fall on your, rinsing off your fluids as you lay exhausted in the tub. If you could see yourself in the mirror you would see you now looked identical to the mermaid you had seen in the carnival mirror.