[Floyd R Turbo]

Gary, did anything in your system change? Feeding, livestock, nutrient levels, anything at all?

Offhand I would agree with homer that it looks like photosaturation but that doesn't make sense given that it's not a new screen/system. So that's why I asked the question.

my guess is that some nutrient bottomed out or is limiting things and that caused growth to stifle. You might check potassium. You can always add that no matter what, because most scrubbers suck them down. You can try adding iron also. These would just be troubleshooting steps to see if one thing kicks it back in gear.

You could also try to decrease intensity, maybe wire a couple of those arrays in parallel to put them to 50% (or even 3-way parallel for 33%) and then go back to the longer hours. Or a diffuser. If you have a limiting situation, keeping intensity the same and lowering duration may not help because it's an instantaneous thing - the light is too intense so no matter what the duration, the photosaturation (or whatever is happening) is happening instantly, not due to duration. If that makes sense.

Either way, clear off the slime. It's possible that got a foothold because of some other condition in the tank, and pushed the GHA out of the way. To get it back in there, you would need to think like you are starting over, and then intensity is definitely a factor.

So with all that said, I would find a way to lower intensity, I think you have to do that no matter what the root cause of the problem is.