I take it you're not foliar feeding, but just spraying with water, since you say it's an arid climate, is that right? Don't bother spraying anything, water won't help much if it's on leaves unless you're not giving enough water through the soil and it looks like you might be okay there. I don't know if giving fert every time you water is a good idea. cangroit is right about how much water to give. Don't try to over-control the plants. Let the soil dry out such that you and put some of your index finger in the soil and it feels pretty dry, then water. It you have a dry climate and it's warm in the grow room, you might have to water pretty often all the same. Perhaps that's why you water so much already. If you have mold in there that has attracted flies, try to get rid of it. It's not good for you either. Flies are drawn to the chemical odor of decomposing organic material, like dead bodies. You can find a dead mouse or bird inside and that will be enough to attract flies too. It's hard to tell what your plants look like under sodium light. Try to take photos in white light with a few closeups.