How do i know when my saltwater tank is done cycling?

Well I bought my 20 gal tank about three weeks ago and i bought 19lbs of live rock and some live sand... About 3 days later i bought some yellowtail damsels... I never seen my ammonia spike nor my nitrite... like 2 weeks later i bought some clark clownfish.. and my levels are as followed : Ammonia 0 ppm, Nitrite 0ppm, Nitrate 20ppm, ph 8.4 temp 80 and saltwater leverl are 1.028... Does this mean my tank is done cycling? any suggestions?