[spanko]

I am of the opinion that all of the information you are getting about dosing things, while well intentioned, you should disregard unless you can and are testing the parameters yourself. If you can't test for it, don't dose for it. To maintain proper parameters you first need to know what you current readings are, what the usage rate fro them is, what the replenishment rate from you water changes is and only then can you determine what and how much to dose.
I am also of the opinion that the whitening of your coralline is a direct result of the light intensity shift. It will come back in time.
Yes you can gradually increase your lighting back to your 6 hour schedule, but gradually is the key word.