I personally do 10g a week for no reason. All of my levels are stable, have been for the life of the tank. That includes the first 9 months without ever doing a water change. Why do I do it now? Because I figure it can't hurt to change a little. I do have an oversized Euroreef skimmer and a huge sump on my tank. That's why my levels don't swing much. Did I notice any differences? I didn't but my friends swear that they have never seen such long polyp extensions as in my tank. I just do it to keep myself settled in the idea that at least I tried something. Better than not doing anything I figure.

I to use Seachem reef mix.

I'm not a coraline algae freak so I can't answer exactly why my coraline disappeared. When I had low intensity flouro bulbs coraline grew like it wanted the entire tank all to itself. It would grow in clumps and would also plate like an SPS cap. When I turned on the halides the coraline disappeared. It quickly receded from my back glass. That glass was completely covered in coraline now it's crystal clear. So with regular water changes, doses of Bionic, Sea chem reef complete, Reef Alk, and Kalk it still doesn't grow. I have heard it grows much better in "yellow" (below 10K) light. I've never researched whether the statement was true or not.