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You may need to take a picture of it out of the bag, or planted in your tank. It looks like possibly a Melon Sword.
Also, just for the future, you may want to research the plants before you buy them. I think you lucked out here because that plant seems to be fully aquatic. But most LFS and Chain Stores have the bad tendency to keep semi to non aquatic plants and sell them. And if you put those in your tank, they will decompose and throw off your tank! Just food for thought
It is a hemigraphis exotica purple waffle plant. Acanthaceae. She told me what is was but I just wanted to make sure she knew what she was talking about. I have never seen one. It's in a bunch. Here it is in my tank.