Hello, i looked some pictures over google. But i cant dedice to problem. 3th week of plant. 250w mh light nearly 22hours open. Plagron lightmix soil with extra perlite. i cant find good quality ph meter. So im not sure from ph. Lastly i didnt give it to any nutrients till yesterday. first time i gave advanced nutrients connoisseur grow a+b yesterday. thanks.
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Your nutes should be pH Perfect so I wouldn't worry about pH unless your water is really hard or something. I've used those before but just use the pH Perfect 3-part for the last few years. I just got some of the AN Jungle Juice nutes that don't have pH Perfect but are just like the older 3-part nutes in their N-P-K formulation. I'm growing in ProMix HP that has no nutes but mixing in 3 different manures plus other organic inputs to have a semi-organic grow. I only use reverse osmosis water so I don't have issues using the very hard tap water we get from a dugout on our property. 400+ ppm and 8+pH so nasty water for indoor plants.
You won't really need a ppm pen either if you just go easy on the nutes. Use about 1/4 max strength at first then as the plant gets bigger you can use 1/3 - 1/2. Better to see a little yellowing on the older leaves to tell you when to increase the dose than to overfeed it and then have burnt plants.
I'm not sure how many nutes were already in the soil you are using but your plant looks pretty healthy so it must have some if you just started feeding yesterday. That can cause problems when supplementing with bottled nutes so just water the next time before feeding any more.
PPM and pH pens are a must have for hydro growing but not so much when growing in soil. Especially when using pH Perfect nutes as they work fine thru a large range of pH conditions.
Good luck and good growing!