I think common sense would dictate that diffusers wouldn't harm the fish or coral. I went snorkelling in PR and can assure you that as the waves break against the coral reefs, there is mass quantities of bubbles circulating continuously around the coral and fish. It is also revealing that the brightest and densest populations of coral and fish were at the base of the coral heads where the bubbles from the surf were the heaviest. IMO the warning against diffusing a SW tank is an old myth that got picked up as a truism without much thought-"it's just what I've always heard", therefore it MUST be true.