They can be VERY slow growers. Some more than others. I have a couple of quick growers but it seems the more colorful they are the slower they are. Some of my colonies grow about 1/4 an inch in a branch a year. How do I know that exact amount? Because they've been spawned in my tank. When the colonies get large that growth seems like a lot because it's over a larger colony and 1/4 an inch can put a lot of girth on a colony with 50 branches. But yeah, very slow for many.
Best way to tell slow grow vs "fast grow". (The fastest one I have grows about 2 inch a year)
Color: Usually (not always) the more colorful, the slower it grows.
Polyps: Huge polyps few in number with long tentacles tend to grow slower. These seem to do better if supplementally fed.
Small polyps in moderate numbers grow slower. Where as small polyps in thick clumps grow faster.
Med/large polyps in dense clumps usually grow much faster. This seems to be one of the biggest indicators. Dense polyp population means faster growth despite all other characteristics.
As for placement. Mine seem to grow more if they are placed close to the surface with good flow. My tank is only 15 inches or so, so it doesn't take much to be closer to the surface. However colonies placed on the highest rocks seem to outgrow those placed lower down. Keep in mind, many of these stereonephthya species grow close or above the tide line and are often exposed to direct sunlight when the tide is down. While I've never collected these corals in person, when I've imported colonies they have all the signs of a coral that grows very close to the surface, including macro algae growing alongside them as well as what looks like plant roots tangled in the rocks. A few have come in attached to a combination of rotted wood and rock (not coral).
wow I am amazed how slow of a grower they are. beautiful tho.
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