I have done much research on the subject (extensive readings, amateur pseudoscientific experimentation). As mentioned earlier, most SPS will take only the smallest particulate food. Many people dump lots of food into their tanks that while feeding the local pods and bacteria, are not consumed by corals. I have seen my SPS take food with a particle size of less than 200 microns being ideal. Just because a coral retracts its polyps when you feed it doesn't mean it is eating. It seems that in the wild, most acropora species will consume only the smallest of zooplankton, perhaps also able to utilize tiny particles of fish waste for nutrition as well. Of course, the vast majority of nutrition is gained via photosynthesis (zooxanthellae), and do fine in captive systems without supplemental feedings, supporting the idea that over time, a captive reef will supply enough supplemental nutrition on its own.