Septic tank?

How do I stop the septic tank stinking? We built a new house & septic tank installed just over 2 years ago. I don't use cleaners etc. that are not suitable for septic tanks. I have tried adding recommended bacterial agent purchased for the purpose of solving problem. Tank has been emptied 3 times in an effort to rid smell. It seems to be coming from the vent pipe beside the tank. Help?!?