What pest is causing this and how should i fix it?

Do not spray your plants with any sort of alcohol !! This is ghetto science! I see a magnesium deficiency, but I really can't see an insect problem at all.

Plant Clovers as a companion, most pests prefer to eat clover over cannabis.

Possibly root aphids, they suckle on the chloryphyl before its transported up the stem leaving empty yellowing which can easily be mistaken for nitrogen. Nitrogen being mobile within the plant also means it will take nitrogen from older growth first to make sure the parts of the plant matter closer to light/survival are least effected. First check ph if you can. Nitrogen prefers higher ph so maybe if the medium is low ph possibly disrupting uptake, but because you mentioned Check back of leafs and soil for critters. Identification is important. If it's aphids you can use isopropyl alcohol or rubbing alcohol with a squirt of dish soap and some cayenne pepper, this kills on contact and I find to be a cheap and effective solution. Be careful tho, of all the nutrients, nitrogen being the most used/abundant it is also the most sensitive to toxicity. Good luck 👍