Paly/Zoa Question


Reefing newb
So every time I get a new paly or zoa, the stems are generally pretty short. The zoas tend to be so short you don't even see them.

However, after a while of being in my tank, the stems seem to grow and become kind of long. The question is why? Is it a flow issue? A lighting issue? They look fine otherwise and they maintain color and propagate.

I have a 40 gallon tank with a 150w MH and 2 24" PC lights.

Any thoughts or suggestions are appreciated!
Might be an extension for light. Where abouts do you have them? Low, high? Amount of flow? Are they dying, or are you just curious?
All the bulbs were just replace March/April 2013. However, prior to that they were around the 1 year mark, which I'm pretty sure is too old.

I have them very high, on frag racks just below the water line. The flow at the top is fairly low. One of the racks sits over my MP20.

None of them are dying, just growing lone. I'm just curious.