New saltwater tank not cycling

Hello, I'm new to saltwater aquarium and have a 165 gallon tank that has been running for about 2 1/2 weeks now. I used live rock, live sand and Catalina water. With all that I never saw any ammonia or nitrites, but it appears that I have around 20ppm nitrates. Does that mean my tank already cycled? So, I'm not quite sure what to do. I'm getting mixed advice from the LFS. One person says that it might just be residual and another says that it has completed the cycle and to wait for algae to start to grow. Curious what you guys think. Oh and in addition about 4 days ago per the advice of one of the guys at the LFS I put a ziplock bag amount of skimmer waste from the LFS coral tanks into my tank to ensure that it does cycle and get a good amount of bacteria into the tank. Thanks again in advance!