Basic plant problem. need advic3

I did search before i posted this, and i found a bunch of different answers, and some tht didnt have answers. My leaves on one of my plants are starting to curl downward on the tips. also like 2 of the leaves on the top of the plant are starting to turn a lil yellow. but in the middle of the leaf! not the edges.
could my light be to close?
400watt HPS about a foot away from my plants. give or take a lil inches.
wat could be causing this
thanks guys

how old are the plants? What nutes r u feeding/how often?

idk how old my plants are. maybe like 4-6 mnths onld. i use some basic nute mix thats really dillutted in the water. i use it once every 3 waterings.

4-6 months?!? wow.they must be monsters.is your light well vented.if not you may put in a few more inches far.