Too much light for corals?

[Brucechm]

I'm running the 24" Orbit IC Pro LED light system on my 28 gal bow front reef tank. I have the lights set to run 8:00am to 8:00pm with 30 min sunrise & sunset and 4 hour moonlight. I have noticed that by 5:00pm to 6:00pm the corals start closing. I'm not sure why they close up like that 10 hours? Could I be running the lights for too long? 12 hours on plus 4 hours of moonlight might be too long?? Any ideas.....

 

[Ron Reefman]

It's possible. The zooxanthellae inside the coral only due photosynthesis for 5 to 8 hours. They need an hour to get started and then run for 4 to 6+ hours. It's a genetic thing. They have been getting 5 to 8 hours of strong sunlight on the reef for tens of thousands of years. Before 8 or 9am and after 3 or 4pm the sun is still bright, but the angle of the sun going into the water is so steep that the PAR is too low and no photosynthesis takes place.

Maybe a longer sunrise and sunset would help?