Help, new grower- leaves getting brown spots that spread and leaves dry out

Strain: white ice
5 weeks old plant which is about 10 inches

lights: cfl, 240w, 6400k
grow medium: common organic soil
humidity: 35-55%
temperature: 28-32Celsius( 80-90 Fahrenheit)
nutrients: bio-grow of Biofrizz
exhaust fan and small fan inside the tent for air circulation

the last two weeks noticed some brown spots on leaves that seem to spread and tips of leaves become dry and spread making leaves becoming crispy. Mainly on the lower leaves but just spotted a few brown spots on the higher leaves!
Tried to adjust my ph but the water running from the bottom of the pot was below 6.0. On the last watering I did yesterday, water that run from the pot was 6.5 ph

I don't know if its nutrient burn, heat stress or spider mites

Im no expert but i had problems with coco.. so i did alot reading up on plant problems. what i found helpes me make out the problems . there is a leaf chart on google under marijuana plant deficiencies under images it shows almost all the deficiencies it helps find out the problem so you at least know what your looking at and not guess. About 12 years under my belt and i still learn every day something new..