After the cycle. Just don't over do it or you will cause a spike in your levels. Snails can generate quite a lot of waste. Add a few things every week and you should be fine. Start with hermits and a fish.
After your cycle, I'd suggest doing about a 25% water change. Test your water again 2 days later. Then, you can add a few cleaners. See how they do for at least a week or two, then your first fish can go in.
