My opinion is to wait at least a week.. test daily. If after a week you still do not see any measurable signs of ammonia or nitrite then it may be ok to put in 1 hardy fish, like a clownfish. Stay away from damsels... they are cheap and hardy but a big pain in the butt. The longer you can hold out putting a fish in the tank the better the chances of a good outcome will be.. 1 week in minimum... 2 weeks is better.. 4 weeks is ideal even if you never got any bad readings because it gives the tank time to settle and get the bacteria population growing.