Dirty sandbed

[fireguy98]

So tank is one year old been very stable for the entire year no issues till last week. My sand bed has started to get patches of what I think are diatoms. Lights are 2months old I run two bags of chemi pure elite. Parameters are cal 450 kh 9 mg 1350 nitrate 0 nitrites 0 po not detected. Sal 1.026 temp stable 80. Water changes water made with rodi unit tds 0. So what gives why now am I having this issue and what can I do to correct the issue. I attached a pic of one of the patches. I just did my 15% water change today.

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