I use plagron regular. Like you said fast dog straight water for first 4-6 weeks then add ur food as soil nutrients running low. So depending how long youve be using that soil for should determine when u add ur food. As for ur problem. How low are ur lights and can u post a close up of under leafs. Imo id of said mite but if uve checked im thinking soils fooked from addin cal mag to a already rich soil ao thts burnt new growth in which case ull need to repot