Over two days my fan leaves have started to get these black dots on the leaves which seems to be spreading, please can anyone give me any advice as to what this is? Many thanks!!!
Update. Is spreading to other fan leaves nearer top of the plant now. Should I start to panic now?
@Mrwangworthy. Welcome to ILGM. 
No need for panic yet. 
If you fill out this support ticket and tell us a bit about your grow we can help you better.
COPY/PASTE: This “Support Ticket” into your forum post.
Answer these simple questions the best you can.
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PH of runoff or solution in reservoir?
What is strength of nutrient mix? EC, or TDS
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Co2; Yes, No
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At first glance this looks to be a Leaf septoria. It is a harsh plant disease that regularly affects the foliage of a variety of plants, including cannabis.
If not handled properly, leaf septoria can be devastating to plants, destroying foliage, stunting their growth, and ultimately affecting the size and quality of their yields.
Pics of leaf underside please. As close up as you can get with detail.