They are probably trying to acclimate to your light conditions if they were being grown differently that you are growing them.

I'd give them more sun. It won't help the old leaves, but the new leaves will be more stout. They will grow fast with heat and humidity.

These 'Black Magics' are over 6 feet tall and are growing in almost full sun!

{{gwi:458394}}
Photo by Boca Joe

My hats off to you if you can grow colocasias in MI. They grow into big plants if they are in the sun, but they need lots of water. I know folks that grow them as aquatics. My experience with black magic is that in the hot FL afternoon sun it will wilt if you don't have a hose running on it. I keep mine in the shade and water every day.
john