We are in the SPS section, so I am going to limit my responses to just acropora. Feeding does nothing, but it also does not seem to hurt unless you already have high residual levels of N and P. Each new food bring hope and optimism over products of the past that have become punchlines of the current reefers, but none of them seem to offer any tangible benefit in our tanks. All that Reef Roids (3 containers of it that I won in a local club auction) did was to raise my P where I had to export more of it. Acro power did nothing either. The friends that I have that grow coral the fastest and with the best color do not feed anything. These are all anecdotes, but they are as helpful as some studies of creatures in/near the ocean - my tank has filters nor I do not have access to vast varieties of live ocean zoo plankton.
The acropora can catch and consume non-matting cyanobacteria that are waterborne - they can keep nearly all of the energy in the bacteria. They catch and assimilate through their slime coat and not the polyps. The issue is that these bacteria need a lot of surface area to multiply and our filter systems keep their numbers low. Another issue is the low surface area or most acropora to be able to catch these bacteria - especially frags. Carbon dosing, ammonia and some types of amino acids can get these numbers up, but the carbon dosing and aminos have other unintended consequences, so dosing these is not for those who do not understand everything involved. This is real science and also works in tanks - a good blend of academia and real-world. Even in the ocean, this is how most acropora get any supplemental "food."
Lighting is still provides the vast majority of energy to the corals, yet I am amazed how so many stray so far away from nature, full spectrum and high intensity with edge-case reasoning, false-equivalent "studies" and just message board parroting. If people spent as much time trying to get a good spectrum and output as they did worrying about putting supplements in their tanks, nearly all would be a lot better off. This is overly simplistic, but all of the people who you see adding T5s and getting much better results have done just that, even if they did not think of it as a "food/energy" thing in this context.
Ammonia/Ammonium dosing with high quality/quantity lighting is likely the best thing that people can do if you mush supplement something. You can get the nh[3,4] from just feeding fish more.
