I had this similar problem when i first started out in 33 gallon tank, it would stink and the water smells like rot. I solved it by changing the water more frequently, vacuuming the gravel and cleaning the sponges inside the filters. After a week of small daily water changes and not over feeding my tank, it now smells like fresh aquarium water.
how often do you change the water and does your substrate have lots of food lying around?