Deficiency, fungus or pests?

This is a Power Plant 3 1/2 weeks into flower

Soil - All-mix
Nutes - None as yet as all-mix has sustained her, miracle grow ready for when she needs it.
Lights - 2 x Envirolite
Temp - 63 night 72 day
R/H - 50 - 60%
Ph run off - 6.8

This girl has been flourishing beautifuly until a few days ago. The problem is starting at the base of the plant and is effecting fan leaves. The leaves yellow and develop spots (along margins mainly)
I repotted as she was a little pot bound.

A few factors that may be clues are:

There is no fresh air in my grow space. I have a flowering and vegging closet which both have fans in and out. Both closets are in the shed though and the only air they are circulating is the air in the shed. I know how bad this is and when I have some cash I will get fresh air coming in.

I have little black flies that appear from the soil when the plants are disturbed. They are stupid flies that sort of tumble round in the air very unco-ordinatedly.

I thought it was a deficiency as I haven't ferted yet but I just dunno.

The overall appearance of the plant is a bit dull and that tip scorch combined with the leaf you show seems to me like a combination of too much water and not enough root space decreasing the plant's ability to uptake nutes. Combine that with insufficient air flow, and you are really hurting your potential yield and the health of the plant.

More holes in that bucket to increase oxygenation in the root zone.

Flush to remove any toxic salts in the soil, and immediately feed with a bloom fert that contains micronutrients, including calcium. Mix it at half strength the first feeding.

Significantly increase rate of air exchange- even if that means leaving the door open while the lights are on- every little bit helps.

Do the flies comeout when you water? Probably fungus gnats.