Alternatively, you could start with two tetra and slowly increase the number every five days or so...
I wouldn't. Most tetras are pretty sensitive to water quality and need an established tank.
Actually, any traceable amount would be plenty enough. Two snails in a 10g can kick off a cycle, I know from experience. In this case though, I was under the impression there was an ammonia source. I'm not sure why I assumed it, though, I probably shouldn't have. :wall:
Sorry I meant to add "in two days."
I suggest reading the sticky and doing a fishless cycle. It's your best option.