I have a new tank that has been set up for a month and a scrubber that has been running for 3 weeks. I started with rock and sand that had been cycling in a plastic bin for about 4 months. I have two fish in the tank and have been feeding them pretty well. How long will it take to start getting green algae growing on my scrubber? so far....nothing at the three week mark. I don't have any growing on the rocks in the tank either but do have cyano doing fairly well. The last scrubber I installed on an already established tank and saw growth in the normal amount of time.