Why are my neon tetra's touching the surface of the water with their noses?

I would say that they are either hungry, or they need oxygen. I would start by feeding them a little more food each day. If this does not work, then I would consider purchasing an air pump, with an air stone. Keep in mind that if you decide to get an air pump, to get a regulator valve and a check valve as well. The regulator valve will allow you to control the amount of oxygen that you are pumping into the tank, and your check valve will prevent the siphoning of your tank in the event of a power outage.