What are this brown spots on my leaves

Hello all. I was hoping you could help me out here. I am growing a few plants and a couple of the older plants have brown spots on the leaves. They actually started out kind of weird. They started as silvery spots that eventually turned yellow and are now brown. Since I have 6 younger plants(2 weeks) growing in my hydroponic system with the older plants I do not dare to feed them full strength nutrients quite yet. I have not seen any spots on the new growth yet. I don't care a whole lot about the bigger plants because they are just bag seed but I hate to see them go to waste. I ordered the younger plants online from a couple different seed banks and they are growing great. They are feminized White Widow Skunk and Big Laughing. Anyways what could be causing these spots. They are fed by drip system 3 times a day. I also give them 20 hours light under a hps 430 san angro light 12 inches from the top of the plants. The temp stays at about 80 degrees. Its not burning the leaves because I have a fan constently blowing on them. I also have a good inline fan exhausting the room every 15 mins. I keep the humidity at around 35 to 40 percent. I think that I'm doing everything alright but still hate the thought of seeing those damm spot. If anyone has any ideas please let me know. thanks

i had the same problem it looked like nute burn but i didnt use any nutes my light i think was to close i moved it up the problem for me didnt get worse so in my case it was the light i think i checked for bug had none i also have a fan blowing on them

check for wee crawley boys under the leaf