While the focus of most of the discussion revolved around water quality which is a part of the issue and a big part at that a forgotten part is light quality. Most stores sell a simple setup of No's because of the price and shock factor for customers when they see what PC and Halides cost. Most good growths of coraline algae can sustain themselves in a healthy tank for up to 2 months with almost little to no lighting. The reproducing however relies on both water quality as well as lighting. With purple coraline algae is also a hard green algae which can also sometimes grow in at the same time. Add to that red algae as well, not cyano but true red algae. It's not unusual to see any or all these combinations at one time in a new tank. Often times the small pink dots you see appearing on your rocks will be the first signs of coraline algae.