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How about Mr. Clean magic eraser pads? They will take anything off of anything. Just test in an inconspicuous area to make sure it won't scratch the tank, but I have heard of people using them on LCD screens before...
I have an acrylic tank--never crayon marks--but I am assuming the crayon is on the the outside...try "Goo" or "Goo Be Gone". It removes adhesive, bubble gum and yes crayon.
Chris