Yup, I think you're right, it looks a little tail-heavy to me. Before you add weight to the nose, see if you can get a better balance by moving the battery pack forward inside the plane. If that's not enough you might be able to add a 'heavy hub' to the propeller shaft. If you do add those weights, make sure and coat them with epoxy or CA after you stick them on, so that the fuel doesn't get to the adhesive and make them fall off in flight.