whats wrong with the leaves? wilting slowly curling under

some of the leaves on my plant around the middle are slowly wilting and drying up. my plant is in a 5 gallon pot of soil with perlite mixed in. im not sure how much i should water and how often. im using liters of distilled water for watering. i heard not enough air circulation can cause the leaves to wilt / dry . i open the cover of my grow box while the lights are on and have a oscilatting fan blowing. and i close the cover when lights are off but still keep fan going, but during that time there is no new air getting in the box just old air circulating. maybe this is the problem.