Minn beat me to it... Find a place to move that battery pack forward. Make a spot for it if you can. Usually a couple of small holes drilled to accept zip ties will work very well.

Then add weight if that does not work.. I would not add weight unless you have no other choice as the plane will not fly as well with added weight.

However, in case you havent already heard it before Nose heavy planes fly like crap, tail heavy planes only fly once.