If it smells sulfurous (like rotten eggs) or acrid like ammonia, you may have a problem. As always, water changes are a good temporary solution. If that doesn't solve it right away, you may need to read the article on cycling your tank.
Freshwater tanks normally smell slightly earthy, like dirt or grass. If that smell gets too strong, definitely do a water change. In general, any strong smell coming from your tank means some kind of imbalance is in effect. Do a water change (at leats 20%) immediately, and try to figure out what your problem is. If it's just too much algae or nitrate, reduce feedings. I used to feed my fish only about 3 times a week and they were fine.
Good luck!