[dj2606]

http://www.theplantedtank.co.uk/algae.htm

My $.02 is go to your LFS and bring some water from your tank. Ask them test do a complete test. Most LFS do these tests for free.

Next instead of cutting your lighting down to 5hrs. Reduce the amount of light. For example, in your 8 hour photo period maybe have 4 hours with only 2 lights running. Then 2 hrs with all 4 lights running. Finally, the last 2 hrs cut back down to 2 lights again.

Also:
What type of filtration do you have.
Do you have any other source of water movement (powerheads)?
Heavily planted with what?

 

[dj2606]

Well going to get the water tested will show if anything is out of the ordinary. Only testing the PH doesn't give the whole story.

Telling us what the water parameters will help diagnose the issue so the problem doesn't keep reoccurring.