RTN/STN or lack of light?

[DurTBear]

I have a couple sps colonies that appear to be dying near the base. I was just wondering if this is a case of RTN/STN or simply because they are shadowed now. The area is pretty dense with sps so there isn't much light getting through to the base of these colonies. Here are a couple pics. Any thoughts on what's happening? Does anyone else experience death at the base of sps colonies?

 

[A_Lee]

Durwin, I've observed that in a lot of larger SPS colonies. As the colony grows larger, the areas of the colony located near the base die due to lack of sufficient light. It even happen to a piece of digitata I had in my frag tank. The side of the digitata frag that was on the opposite side of the light source lost a lot of color, and almost all the polyps
Looks like things are growing very well in your tank
How is your Ca, alk, and Mag?

 

[DurTBear]

Thanks Ant. That's reassuring that you've seen that as well.

My Ca, alk and Mag are 440ppm, 10dKH and ~1400 respectively. I think my Mag might be a bit high as I needed to increase it recently. Might have overshot it. But I noticed things were precipitating out of solution onto the glass when I dosed alk and I read that you have to increase your Mag to make sure the Ca and alk will stay in solution at a high concentration.