One other thing to add to the mix - if you change in the range of +40%, then many of your corals that are near the top are out of water. For a short time, this does not pose a problem, but if you're slow to refill would this cause added stress? Especially if the water change was initiated by a problem? I would think that with such large water changes the MH lights are turned off so that it doesn't cook the corals at the top.
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