As already stated, there's something that you're not doing that is causing a strong smell. Poor filtration and lack of adequate water change is typically the main cause. I don't use activated carbon nor any of the manufactured chemical aide and none of my tanks have an offensive smell, but rather just an "earthy" smell if you put your nose right over the water.
There was one time when one of my tank did have a foul smell.I discovered it was from some of the pellets getting splashed up, during feeding frenzies, and collecting between the glass covers and tank frame/brace. Once I wiped that clean, no more smell.