[silver616]

I believe that I read in another post on here recently that someone had good success with using vinegar to clean their tank. If it were me, I'd try to empty out the tank and give it a good cleaning with the vinegar. let it air out for a couple of days, and make sure the smell is gone before putting it all back together again.
If the sand you put in after cleaning it previously is from the same batch as the sand that you had in before, there's a chance the batch of sand could be "bad" and contributing to the odor.
Also, getting the humidity back into the "good" range may help as well...when the humidity's too high, food molds, sand molds, and it creates some nasty smells.
Good luck!