Do you mean that you started this tank cycling a week ago? If so, are you doing a fishless cycling?
When adding fish, the basic idea is not to overwhelm the bioload. In otherwords, the more fish you add at once, the higher the bioload. I added 3 fish per week once my tank was cycled, and only if my ammonia and nitrite remained at zero. It's worked beautifully for me. Oh, and mine are relatively small fish in the tetra family.
What kind of fish are you planning on getting? That might make a difference as to how many you can add at a time, and how often.