Your fish tank could smell from a number of reasons. I think the most common reason is when people feed their tank, the fish are so used to feeding off the surface that they splash the food back up onto the lid of the aquarium. It gets on the rim, it gets underneath the light, and it cooks under the light. Food is animal protein, it's nasty; it's going to break down and it's going to smell. You always want to, when you clean your aquarium, you want to clean the top, wipe everything down, clean up all that leftover food. That's something you should do every few days if you don't want to smell any of this stuff.
Running carbon in your filtration system will help to filter out a lot of the impurities and a lot of the odors. You've heard of carbon-based air fresheners, I'm sure; carbon serves the same role in an aquarium filter. It's going to remove a lot of the impurities, the yellow stains in the water, and odors. If you don't do water changes after awhile, the water may have a tendency to smell. Most of the time, it's usually uneaten food or even fecal matter that's gotten splashed up onto the aquarium. If you have a lot of scum and slime that's been built up around the top, bacteria's going to grow, and bacteria smells. You want to keep the tank clean. When you clean it, just use water. You don't want to use any detergents, soaps, or ammonia; that's going to be very harmful to the aquarium.
Take the aquarium canopy off the aquarium, wipe it all down, be careful, the light is plugged in, so it's electric. Don't work with it when it's plugged in. If you slip and the light falls in the aquarium, you can electrocute yourself. Always keep these things in mind when working with electric and water. Just keep the top of the tank clean and run carbon. Not overfeeding and not overcrowding will help to keep the water cleaner, which will result in less odors. Your filtration, also, if you keep that clean, wipe with a paper towel. On an overflow or hang-on filter, the lip around the top of the aquarium has a tendency to build up some slime. Just wipe that out. Again, carbon goes a long, long, long way. You shouldn't have to rely on room air fresheners. If you are, those are band-aids. There's something critically wrong with your tank. The tank should not smell on a normal basis.
We service aquariums, and they have them in CEO's offices, the craziest most beautiful homes, and if they smelled, we would never be able to keep these tanks long-term. You should not have to live with a tank that smells. Something's wrong with the tank.