Gnats do no real damage. It's not necessary to spray poison on the plants just for them. Usually they hide and breed under leaf-litter. (clean the crap from the soil surface)

Check my signature for my experiences in dealing with the heat...

Cannabis prefers a wet-dry cycle, but don't let them completely dry. Especially in smaller pots which dry quickly.

Most liquid ph tests are unusable for runoff, as the color-coded test results are skewed by the tinting of the runoff itself. A ph pen is a great addition to your growroom, but avoid Hannah. (I use Milwaukee) If you get a ph pen, get the storage solution and the calibration stuff from the same manufacturer.

Looks like it was time to transplant a while ago.

Have you taken a look at Bio Grow's weekly feeding schedule? Is a better way to feed the ladies than being on a random schedule. Not the easiest schedule to follow, but if you need clarification, give a holler.
Bio Grow Feeding Schedule