Sounds like you're on the winning end of a mini-cycle. Adding a few new fish at once will usually result in a temporary spike in ammonia, but your bacteria have adapted well and grown in your filters to accommodate the waste. In a few more days, you'll see the nitrite go away and the nitrAte rise. Then you can return to your normal water change schedule to keep nitrates as low you want them. For now, just watch the ammonia (change as necessary to keep it at 0, should it test positive) and let the bacteria eat the nitrIte. What size tank? How much biological filtration are you running?