Need help! Coco 50/50

i noticed that one of the plants has developed small brown dots around the lower two leaves ( very small brown dots) and there is a small amount of yellowing on one leaf indicated in the picture. Im not sure what to think as all the other plants look healthy. I feed only when the sand on top starts to dry and I am currently feeding 70ml of water till i see run-off.Also when should I repot into a larger pot? Would greatly appreciate your feedback!
Thank You!

Grow Info:
Temp : 21.3C
RH : 56%
Medium : Coco (50/50 coco/perlite)

Water EC : 600ppm
PH : 6.1
Container : 250ml (solo cup)

Coco should be fed to run off daily.......that could be your problem.
I would transplant.
Why the sand?
What water are you using?
Are you using calmg?
Never let it get dry and never use plain water in coco.

MickFoster said:

Coco should be fed to run off daily.......that could be your problem.
I would transplant.
Why the sand?
What water are you using?
Are you using calmg?
Never let it get dry and never use plain water in coco.

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I use the sand to prevent fungus gnats and currently using ph'd water (6.1ph) with a ppm of 600. The nutrients I use have both calcium and magnesium. So I dont add additional calmag. Should I remove the sand?

When I asked about the water.......I meant tap water or r/o?
It's doubtful you would have a fungus gnat problem while in solo cups.
If you use r/o you will need to supplement it with calmag........probably ok with tap water.
All nutrients have magnesium and calcium, but sometimes it's not enough for cannabis. What nutes are you using?

how old is your plant? 600ppm seems like quite a lot of nutes for a plant that small it may be getting nutrient lock'd

Larry27420 said:

The nutrients I use have both calcium and magnesium.

Click to expand...

What kind of nutes and amounts of each?