the only way is through water changes. Is this an established tank? do you mean 8ppm if so this is not high for nitrates, although it may be odd for your tank. Do you mean nitrITE or nitrATE?
Have you tested your tap water for nitrAtes - you may be surprised....
How are the fish - unless they appear to be suffering then its not really an emergency and you should be able to sort it out with a several small water changes....
my tank is 48"x24"x24" with about 24 fish in it altogether, yes i did test my tap water and that was fine, if i do a water change how much water should i remove from my tank.