Well, you're only the 4th or 5th person now that I've heard from that has experienced what I call "whiting". This is a strange phenomenon that seems to be associated only with LED scrubbers. As I hear more about this I continue to develop a theory to explain it, but so far I've started to see a pattern.
Something essential to the algae is bottoming out and limiting growth to a point where photosynthetic production literally stops, and all the incident light just bleaches the algae out, and it seems to happen rather fast.
Almost every scrubber I run keeps the nitrate at zero. It's not really dead bottom zero, because there is always waste being produced, but over the long term with a scrubber running on the tank, it remains almost undetectable. So I don't think that is the problem.
I've also had tanks running with phosphate at zero, and I'm talking 0.00 on a Hanna Phosphate checker (not phosphorus) but the same tanks usually have days when I test 0.02, 0.04, etc. Still low, but not rock-bottom.
I haven't heard of anyone testing their water for other elements that might be affecting growth, just these two. So it might be something else.
Here are the cases I have come across:
A tank running a scrubber, biopellet reactor is added with a large quantity of pellets. Within a few weeks, a whiting incident occurs.
A large tank with low bioload and low nutrients is running only a scrubber, N + P are already low, scrubber is being run aggressively (high flow, long photo period). Whiting occurs
A large tank with moderate load running a large skimmer and mechanical filter (DD system) is lightly fed, no N & P to speak of, running two 4 cube/day scrubbers (one waterfall, one upflow) with long photoperiods (18 hrs/day) and whiting occurs.
The pattern here, that I see is:
Low nutrients in water
Not a lot of issues with "bad rock"
LED-based scrubber with very high intensity
Very aggressive scrubbing, meaning LEDs are strong, photoperiod is long
labas39, would you say that you are falling into this category?
Do you have any frame of reference between where the screen was growing just fine, and then it was stark white?
The fix, that you can try, is to reduce the photoperiod and maybe back them off a bit, like 1". If you were running 18 hrs/day, that's a problem. Strong LEDs don't need to be run that long. Also your screen got wiped out so you're back to a maturing stage and my recommendation is no more than 9 hrs/day or else you risk photosaturation (not enough algae growing, so there is too much incident light compared to the nutrients that can be absorbed by the small amount of algae.
Whiting appears to be the extreme end of photosaturation. It seems there is some kind of "hard line" that you cannot cross, or the screen just dies off.
Come to think of it...I have had this happen to me, but the instance was when I was gone for a week on vacation, and a fish got stuck on the scrubber intake pump so it was only getting 5% flow for about a day. The T5HO lights cause the growth to turn white in the absence of flow, but I figured that is just what happens when you have zero flow and tons of light. Apparently, this can happen when flow is actually present as well.
Unless no one is reporting their flow outage to me...that would just be too coincidental.