What is the problem with my plant please help? Thank You

Hello,

I am posting a picture that is pretty hard to see but on the one leaf (far left but not theo ne that barley showes nothing its the big leaf to the right of that) it has brown spots on it, u can kind of see them, also the leaf is turning yellow.

What could be wrong?...i have read it may be Ca deficiency, but I cant really tell, maybe too much nutes, or to little i dunno? If you can help I would really appreciate it, its killing me not knowing whats wrong lol Thank you

i forgot to give details

Light : 400w MH
medium: rockwool in hyrdoton rocks in a hydro system
PH:6.8
Humitiyu: 38%
age:3.5 weeks
(nutes) i think i have been to careless i gave my plants a full dose of nutes bc thel eafes were yellow and i thought that meant nitrogen deficioncy and the half dose wasnt making it greener

in hydro your ph should be between 5.6-6.2.

Revised: The Complete Guide To Sick Plants,pH and Pest Troubles - PlanetGanja.com