to monitor nute levels you would need a tds meter but i think you already knew this, and you were really asking a good brand of tds meter someone with a little more knowledge on this subject to help you out. as for your problem yes they do look nute burnt but the real worry to me is you said none ventilated so you do not have fresh air coming in? i mean they look great! but a plant can use up the 350ppms of co2 in the air very quickly do you keep a door open? have you ever had mold issue? i am very interested in that. man they look great, i would flush and get ready to harvest.