2000 Altima SE Gas Gauge

I have a 2000 Nissan Altima SE with a 2.4 liter, 4 cylinder engine, and about 75,000 miles. The gas gauge does not function correctly. It still works slightly but it is very hard for me to tell when my tank is full or empty. When my tank is empty, the needle is on half full. I think this problem could be due to the fact that I left my air filter box open (my friend told me I would get a little more power and I did). I don't know if this is the answer to my problem, can anyone tell me if it is?