Yellowing

The leaves on my plants are yellowing and drying out . I'm using 400w hps . The light is around 1ft away with a 9'' oscilating fan in tent . This is my second crop and my last one was the same . I got a good harvest last time . Am i overwatering ?? Just started feeding them today as they're only small . Last time it stopped when they got bigger . Any ideas ??
 
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