I agree that this is something that can't have a definite answer. Lighting, water parameters, feeding and stability are all things that we know cause growth. But as pickupman said, some corals grow and spread faster than others. When it comes to acans, you will need to keep your alkalinity and calcium at the correct levels, but they also will grow faster by feeding them. Mysis and cyclopeze are good. As far as sps, I have seen the fastest growth in my birdsnest corals and Monti caps. Both of these are both considered easier sps corals, because they are not as demanding as acros. There will times where you can literally see the growth by the day. And another good way to gauge growth is to take pics and and date them, or videos. As far as softies like mushrooms and zoas, they can be incredibly fast growers, but this is usually when the water has a little more nutrients in it. That is why I think my zoas have Been slow growers, because of the use of bi-pellets, and the lack of nitrates and phosphate.
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