I know you have waited forever to get this tank to cycle.
And I don't want this to sound like some "voice of doom" post, but I'm not sure how much ammonia those snails created, you said you didn't feed them, didn't you?
Just to be sure, I think you should do as WebCricket suggested and check the tap water for Nitrates. Since city water was once ground water, and can fluctuate from day to day, even that may not be foolproof.
If the snails did bring about a cycle, keep in mind that the bacteria colonies are pretty small. Their size is directly related to nutrient source.
Add fish slowly, and bear in mind that you might go through a mini-cycle when you do. It won't be anything terrible, and you already know how to handle it, since you have cycled before. It won't last that long either. Actually, each time you increase bio-load, you will see this to some degree, it's just the bio filter increasing to match the new bio-load. The more established the bio-filter is, the less noticeable it is.
