Need help! Coco 50/50

[Larry27420]

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)

 

[MickFoster]

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.

 

[Larry27420]

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.

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?

 

[MickFoster]

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?

 

[eyderbuddy]

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

 

[TintEastwood]

The nutrients I use have both calcium and magnesium.

What kind of nutes and amounts of each?