I would also say to low nutrients.Stop the vodka,feed more,reduce full lighting time.
No. The issue is not low nutrients. I'll say again, low nutrients causes a pale, washed out appearance, aka bleaching. NOT brown. The fact that the OP has measurable phosphate should be an indication that it is NOT due to insufficient nutrients.
Brown = excess zooxanthellae = too many nutrients or too little light.
Excerpt:
"You all know that nearly all the corals we maintain in our aquariums are hermatypic and that their tissues host symbionts, the zooxanthellae. These symbionts are unicellular algae (phototrophic dinoflagellates) and, as such, are able to transform luminous energy into chemical energy in order to manufacture sugars from inorganic CO2. A part of the products synthesized by the zooxanthellae is transferred to coral cells where they are used as fuel and a carbon source for the coral's own needs.
This particularity is advantageous for us as it helps us maintain animals in our aquarium without the need to feed them much. On the other hand, it has the inconvenient of turning our corals into brown animals in certain maintenance conditions. There are multiple species of zooxanthellae - in fact tens of species and sub-species - but they all are golden-brown, a consequence of the privileged absorption of the blue/green wavelengths by the photosynthetic apparatus - brown is in fact a very dark red! The visible color of a coral will be the result of the mix between the colors of its pigments, the dominating brown of the zooxanthellae, and the respective densities of these two components. The whole will depend on the two organisms and the subtle laws governing the coral/zooxanthellae symbiosis. It has been shown recently that a third organism - a cyanobacteria in fact - was a participant of the symbiosis in Monstastrea cavernosa and has an impact on the final color. Today, we do not know whether this type of association is very common.
The coral polyps have two metabolic paths available for their growth:
•They can use the zooxanthellae, therefore, indirectly, light and CO2. Under those conditions the coral nearly turns into a phototrophic organism
•They can use their own cellular machinery, therefore, like any other animal, it is imperative that they find an external energy source and feed on it.
In their natural habitat, these two possibilities are believed to be simultaneously employed in variable proportions based on lighting conditions, amount of organic matter in the water column and genetic particularities of each species. For most of the animals that we host, it is the zooxanthellae path that is mainly fulfilling the function.
Without any drastic change in the natural conditions, a balance is reached between the symbiotic algae and their host; the coral's color is the result of this balance. If corals are not all brown, it means that the natural conditions allow them to maintain sufficiently low amounts of zooxanthellae in order not to mask the color of their pigments. To say the least, for a coral, a move from a reef to an aquarist's aquarium is quite a change. Turning brown would be the result of a newly established balance through the increase of the zooxanthellae's numbers, and therefore by the masking of the coral's pigments."
Original Article:
Feature Article: Colors by the Thousands - Light, Colors and Corals, Part I
