Brown Spots. Manganese Deficiency?

[V8]

You can also feed with 6PH and messure the ph of the runoff. This will give you an indication of your rootzone PH.

V8

[Jake hammer]

Hi, does anyone know what these brown spots are? The pics are taken one day apart to try and see any spreading. I think it's spreading slowly but I can't quite tell.

There are four plants in an autopot system, only this one is showing this isue to this degree, it's beginning slightly on another one but the other two look fine.

They are 45 days old I and the EC is 1.2 and the ph is 5.8.

Thanks

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Hi
I was about to post with the same kinda issue I've had this problem for several grows now.
The e.c out the tap for me is 0.7.
I keep the ph to around 6 or slightly lower.
With me it starts slowly and spreads. It's never affected yields or quality but something isn't right.
I don't add cal mag really because of the strength of my hard water at 0.7.

[Jake hammer]