leafs got very dry and brown

Hello,

I am growing a plant for about 4 months. It grew beautifully until last week when, the leafs got a little brown. I bot a new fan to reduce the temperature, but with no success. The leafs got dryer and dryer.
I lifted the lights a little more, also to reduce the temperature, so now they are at about 25 cm away from the plant.
The air enters through the black(lower) fan and leaves through the other.
I am using a fertilizer NPK 6-5-6. I used the recommended amount for watering and for leaf feeding.
The plant is exposed to light 18 hours/day.

I have attached a few pictures with the plant as it is today.

Please tell me what can I do to make it better!