sounds like your cycle is finished. here's how a cycle runs:
Nitrites spike as ammonia begins to fall.
ammonia drops to zero as nitrites remain high and nitrates begin to show up.
nitrites drop to zero while nitrates begin to rise sharply.
once ammonia and nitrites are zero, do a large water change 25-35% IMO
and move some life into your tank.
The bacteria that dismantles the nitrates is the slowest to colonize your LR
and nitrates are also the least deadly of the 3 waste biproducts. for fish only tanks, nitrates are "ok" in the 30-40s i think? if you have inverts in your tank, you want to stay as low as possible, obviously zero would be best but is tough to acchieve.
any amount of ammonia/nitrite is bad and a sign of something out of balance.