Brother, when I switched to ammonium, urea and sodium phosphate ALL my corals perked up and really started growing. The corals can use the ammonium and urea directly without having to convert other nitrogen sources.
BOTH coral cells and zooxanthellae (Symbiodinium) can import ammonia/ammonium. Once in the coral cell or zooxanthellae, ammonia/ammonium can be directly utilized for amino acid synthesis (specifically to make glutamate).
BOTH coral cells and zooxanthellae (Symbiodinium), can IMPORT nitrate. Once nitrate is imported into the coral cell, however, it CANNOT be directly utilized for amino acid synthesis. Corals cells lack the enzymatic machinery to utilize nitrate for amino acid synthesis.
Here's where the magic comes in: the zooxanthella absorbs the imported nitrate from within the coral cell (which can't use it). Once imported into the zooxanthella, the nitrate is converted into ammonium (zooxanthellae DO have the enzymatic machinery to do this). Once converted into ammonium in the zooxanthellae, it is utilized to make amino acids. Whatever excess amino acids that are produced in the zooxanthella are then FED BACK to the coral cell. The coral cell can thereafter use these amino acids to build new proteins and grow. Essentially, the coral cell feeds the zooxanthella nitrate (just like you would fertilize a tomato plant) and then harvests amino acids (the tomatoes) that its zooxanthella produces.
today Kev 
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