What was the second plague of Egypt?

Frogs

And the LORD spake unto Moses, Go unto Pharaoh, and say unto him, Thus saith the LORD, Let my people go, that they may serve me.

And if thou refuse to let them go, behold, I will smite all thy borders with frogs:

And the river shall bring forth frogs abundantly, which shall go up and come into thine house, and into thy bedchamber, and upon thy bed, and into the house of thy servants, and upon thy people, and into thine ovens, and into thy kneadingtroughs:

And the frogs shall come up both on thee, and upon thy people, and upon all thy servants.

And the LORD spake unto Moses, Say unto Aaron, Stretch forth thine hand with thy rod over the streams, over the rivers, and over the ponds, and cause frogs to come up upon the land of Egypt.

And Aaron stretched out his hand over the waters of Egypt; and the frogs came up, and covered the land of Egypt. Exodus 8:1-6