Looking for a little help with cyano problem

[Wopadobop]

40 gallon with 30 gallon sump.
feeding 1 cube per day of frozen brine + 1/2 cube prawn roe with .05 gram of A+ spectrum pellets.
screen size is 7 x4 lit both sides by a 27 watt compact flourescent with a full aluminum reflector. 350 gallons per hour flow.

Here is a photo of my screen at 5th day . has been cleaned 3 times. so about a month old.
http://i1146.photobucket.com/albums/o526/Joshua_Clemons/Untitled-1.jpg

All parameters are good( 0 n 0 po4) I realize this is not a good indicator of water quality as the cyano and scrubber are eating it. My question is this. Why is the cyano not faiding away. I have made sure to have A LOT of flow across all areas of the tank. But it seems the cyano just loves my sand. Could it be leaching po4 from the sand now that my water is at 0 po4 ? I did just recently switch from 175w metal halides to a radion xr 30 and it did seem to help. but it has only been 2 days.

Does my screen need more light to get good thick growth. more light? less light?? make it smaller?

appreciate the help boys.

p.s. This is a scrubber only tank and my corals are doing quite well. just cant seem to get a grip on the cyano.

[SantaMonica]

You have food particles settling on the sand, providing carbon to the cyano. Sand will only leach P during big pH swings that dissolve the sand and release bound P. And of course the cyano has no trouble getting N.

Your screen will be growing great. It will take some time and strong scrubbing to beat the settling particles, however. You could point some flow at the sand, up to a point of blowing it away.

[Wopadobop]

I can definately do that. Thank you Obi One! :)