The thing that is very odd to me is that you are getting zeros on all of your tests.
How old are your test kits and what kind are you using? Be sure to read the instructions carefully as sloppy testing can lead to varying results.
During the nitrogen cycle, typically hydrogen sulfide is created by denitrifying bacteria converting nitrates. These cases are commonly released out of the tank by sand sifting organisms before they become toxic to your aquarium inhabitants.
That being said, because you have such a new cycle, its hard to believe that hydrogen sulfide is already being generated. Its common to have it generated in anaerobic areas, such as your sand, especially when you don't have any organisms to move the sand around and release the gas.
What is troubling is the fact that you don't have any nitrates yet which are necessary to generate hydrogen sulfide.
Once you start your cycle, you will see the ammonia rise, and from ammonia nitrites are created by bacteria consuming ammonia. This in turn allows additional bacteria to convert to nitrate which ultimately results in hydrogen sulfide.
If you are testing zero on everything, you shouldn't be having any hydrogen sulfide, especially not to the extent you are explaining.
These kinds of oddities happen while cycling your tank through this process, and it should also be noted that most of the components of the nitrogen cycle are very toxic to fish and other animals. Nitrate and hydrogen sulfide being the least toxic in low concentrations.
I think you should take some water to a LFS and have them test it just to verify your results. If infact you are done cycling, and as such your tank is ultimately converting all of your availabe nitrates into hydrogen sulfide, the snails you added will do the trick to rid the tank of the problem. Until you have this under control, don't add anymore animals.
lol
