Yep 40ppm is considered fairly normal, anything up to 100ppm is said to be within acceptable levels for most fish, of course certain species require it to be much lower, but for an average community tank 40ppm is fine.
Have you tested your tap water, because alot of people, myself included have nitrAtes in our tap water, 20ppm Ive recorded on mine, although it now seems to sit regularly at 10ppm.
Provided you do regular weekly water changes of around 30% then you should be fine and the levels of nitrAte wont rise beyond where the fish are uncomfortable with it.