Coral Colours and Amino Acids

[Reefer Rob]

From what you've posted it doesn't look to be either a lighting, or nutrient issue. 8 T5 bulbs are a ton of light over a 75 no matter what the bulb combination, you could grow SPS at the top of your tank with half that many bulbs.

If your test results are showing 0 for nitrates and phosphates, and you have no nuisance algae, likely high nutrients aren't the issue. Also if you're pulling out skimmate you likely have enough nutrient to grow corals.

Probably it's something you haven't posted yet.

What is your water change regimen? (did I miss this)
Any corals in your tank known for chemical warfare? (leathers?)

Are you corals brown or more of a faded tan color? Brown is caused by high nutrients/low light. Corals tend to go tan and loose color when they are stressed, such as by contaminates in the water, unstable parameters, etc.

One thing I've thought of is the possibility of chemicals being excreted, or trace elements being used up by a large mass of macro-algae in a small system.

Just my $0.02 Rob