a 400 watt hps has more of a yellow spectrum. yellow light breaks down cloroplasts into sugar then starch that the plant uses for food. A metal halide has a more blue spectrum which builds cloroplasts and gives a plant a more green color. You could get a conversion bulb to run on your ballast and it will help but this is not exactly a one step curing process. you have a couple issues that need to be taken care of. Your plant is restorable. I would get some thiamine to help combat the stress. Thrive alive will help. Use one to two drops per gallon of water.

Tell me as much as you can about how much you water
how often
with what
how much of the what
temps day and night
temp of the floor

If the floor is cold put foam under it or foam and an eletric blanket

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c