problem only on the bottom leaves

My problems have been going on for the last few weeks. I noticed the bottom leaves only are effected. I first started seeing the black spots. But then started to see leaves that were either yellowing, wilting, and when touching the stem it pops right off the plant with little effort. Or they are shriveled brittle things that look like they been dead for days but I pull em off every day or every other day. I thought it was mold at first, I sprayed em down with a funguside/insectiside/miteicide, but that hasn't done anything but get rid of a stay nat or two i had flying around while the weather was warm. then I thought maybe I was overwatering them. But now I think I'm under watering them since everything on the bottoms seems to be drying up so fast.

These are pics of one of the suspect leaves. The black spots seem to make the leaf wilt and become brittley dry with in a day or two.

Attachment 280601
Attachment 280602
Attachment 280600


This is what my garden looks like from above. As you can see everything looks healthy to me.

Attachment 280604
Attachment 280603

Aslo on top of identifing this, I'll get better pics tomorrow when the lights are on, should I turn my fan off? I have one blowing on the bottoms to keep air flowing under the canopy.
Thanks in advance for any help!!! This isn't my first grow but first time I had this problem.