It absolutely sounds like Cyano / BGA.
Your problem is most likely the low nutrition levels, BGA is very active when the NO3 level gets to lo.
PO4 is low but not that bad.
NO3 is way to low, a good starting point should be between 5 and 10, and this is the main problem.
Most certainly you are also low on traces.
There is something called Redfield ratio (N/P) and thats the ratio between N and P,this should be around 10. |