I'd added 75 lbs of dry rock to my tank about two months ago as well as a piece of live rock with lots of coraline algae to help seed it. The coraline algae has not spread to the other rocks, and in fact appears to have died off on the rock that had it.
I bought a 4 bulb T5 fixture about 4 months ago and believe I have two 10k and two actinic bulbs in it. My nitrates have been around ~15 ppm, my calcium has been around ~460 ppm, and my dKH has been between 14 - 17 when I've tested it.
I'd been adding a small amount of Kent Marine Buffer each time I do a water change, but I think that may be why my dKH has been high, so I am going to forgo it next time and see what results.
Why do you think the coraline algae died off on the one rock that had it instead of thriving and spreading to the other rocks? Could the higher dKH be to blame? I've tried to research the relationship between high dKH and coraline algae, but the only information I've found seems to suggest that coraline does better
in higher dKH.
Is it possible that the ~15 ppm nitrate is too high for coraline algae?