Dark spot on one leaf...no idea but can't be good :(

These plants were cloned on the 1st of Oct. and I got them a week ago today (Thurs) Today I noticed that there is a dark circle on one of the leaves...I have no idea what this is (too big to be a spider mite) Also I have noticed some gnats in my grow room..planning on getting some azatrol hopefully by the weekend. I've included some pics and if you have any questions about my setup either check my journal, PM me, or just reply I"ll answer as quick and efficiently as possible.

Here's some more info...HTH with someone answering...I"m thinking PH lockup but not 100% sure (although I read a post by uncle ben and it seems to be "spot" on)

1. Are you growing from seed or clones?Clones

2. How old are your plants? owned 1 week 4 week old clones total

3. How tall are your plants? left plant 4 inches right plant 5 inches

4. What type of soil system are you using? Roots Organics

5. What brand/type of nutrients are you using? Roots Organics Buddha Grow

6. What is the Ph of your nutrient solution? 6.5

*A. What is the Ph of your water 7.0

*B. What is the PH of your runoff 6.1

7. Do you foliar feed or spray your plants with anything? no

8. What kind of lights do you use and how many watts combined? 1 4ft 4 tube T5 96 watts total

9. How close are your lights to the plants? 6 inches 5 inches respectively

10. What is the temperature and humidity in your grow space? temp is 79-82 humidity 30-34%

11. Have you noticed any insect activity in your grow space? gnats