Hi there,
Just to start off with, Zebra Snails (Neritina natalensis), cannot breed in freshwater as the eggs need Brackish (part-seawater) to hatch.
What you most likely had was a baby bladder or ramshorn snail, which snuck in on some plants or in a bag of fish.
The majority of snails lay eggs, Zebra snails, Apple snails etc, whilst other species such as Malaysian Trumpet snails, give birth to live young.
Small snails aren't generally eaten by fish as they're too hard to crack into, unless you have loaches or puffers.
They are however, masters of disguise and can sometimes be hidden for days.
If you would like some free snails, pop down to your local fish shop (LFS) and see if they have any, a small group of 3 should be enough to breed your own.
Regardless of what people say, snails are a benefit for any tank, they happily eat any left over fish food (which would otherwise rot) and any algal spores, they just become a problem when you have more snails then litres.
Hope this helps.
