Overwatering is WAY worse than under watering. Let your plant really dry out, check the weight of the pot and then water, and dont water till the pot is that light again, however long it takes.
That fan, its just a normal fan is it? You need extraction, something sucking air out of the grow area. Extract at the top and have an entry hole at the bottom for fresh air to enter. Just a fan blowing the plants wont work, thats just blowing hot air around. PH sounds ok, see if you can catch some runoff from watering sometime and check that PH also.
The light should be full power and at the right distance. You should be able to measure the temp at the top of your plants. Putting a thermometer in direct light will give false readings, best method is a IR thermometer, get one for 10 bucks off amazon or ebay. Or if u cant do that just put ur temp probe in the shade at the plant top level and get a reading then. Adjust light accordingly.