How do i find out the right npk for my plants?

I’m going to be upfront and honest here! The problem I believe isn’t from the lights you are using, one can veg cannabis under CFL’s or even cheap Chinese LEDS , your soil IMO needs help first and foremost here! As stated earlier it doesn’t look the greatest, needs aeration, has lots of bark in it , your ph could be off thus causing , lockout of using poor soil, of the much needed macro and micronutrients! If I were you in trying to save these plants I would immediately change out the potting soil or make your own with peat,aeration ( your choice) and worm castings or compost at a 1:1:1 ratio and go from there! For what it’s worth you probably won’t be happy with the end product if you use weak lights. I was in the same position and thought I could get decent colas from LEDS inside not the case ! They just turned out being little popcorn buds not hardly worth trimming all light and fluffy! Hope this helps and good luck with your grow!

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those appear to be t8’s?? if you could tell us the wattage and T rating. for a standard T8 you want to be about 2-6 inches away. I agree with the above post that you need to do something about your potting medium. does`t appear to have perlite. you will have to be really careful not to overwater.

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I would agree that soil is likely the culprit, again there are many choices. DO NOT use any pre-packaged potting soil that has the phrase " feeds for X months" those products have some form of time release fertilizer that WILL ABSOLUTELY And POSITIVELY wreck your plants.
I mix my own soil, equal parts of clean topsoil and peat 1/2 bag mushroom compost, 1 bag of perlite. Avoid using municipal mulches too much pine and pine bark too much ground up plywood, tree bark is no good for soil mix avoid topsoil that has lots of bark or clay soil content

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See my FAQ candidate about soil testing for pH and NPK

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Honestly they look like they need more light and may even more aeration of the medium w perlite or something

Those are just my guesses but they’re just guesses. Need to know more about what your grow medium consists of and if you’re mixing and liquid nutes into your water. What’s your whole set up? Lights? Hours of light? Nutes and medium?

Will try to help best I can.