When this happened to me, I did a complete investigation with my nose. Try to find out exactly where the smell is coming from. During the cycling process, the water will tend to smell a little foul due to the ammonia as stated earlier, so water changes should reduce this. The smell could also be from decaying food, dead fish, excess waste in the water, so water changes & gravel vac should take care of this as also stated. One unique scenario I had was when I was smelling around when I noticed an odor coming from my tank, I smelled my 2 whisper hand on back filters and noticed the smell was really strong coming from one. There was simply some decaying food caught on top of the sponge of the filter because sometimes I put my pellets near the filter so the waterfall shoots them down quicker. Some must've fallen on the sponge and I didn't notice. So I gave the filter a cleaning and the smell was gone.
It's good to find the source of the problem, rather than treating the symptoms. I have yet to see any water parameter readings, stock level, feeding schedule...