Brown black spreading on goni

[jerallenttu1]

What is this spreading on my goni. I have had it for about 4 months.
Alkalinity 8.2
Salinity 1.025
No3 0
No2 0
Po4 0
pH 8.2
Calcium 360
Ammonia 0

I know the nitrates are low and I have been adding neonitro for several days, but have not seen an increase yet.

 

[MERKEY]

It looks like dead spots where algae is starting to grow.

Try to get a hold on nutrients and keep them stable.