Sagging leaves on just one plants. ASAP advice appreciated

I have 9 plants going as of right now and all seem to be doing ok. A few days ago some started to sag down some and progress that way and the tops of the plants turned slightly yellow but not bad. Currently im sitting next to them and seeing that the yellow tint is going away but the one plant in the picture is still drooping. whats the deal?

BTW 1000watt HPS light and tap water for the first couple weeks(which i know was stupid)

heres the pic of the plant im worried about

https://www.rollitup.org/members/youngonedahungone-albums-my-progress-picture105684-possible-root-rot-over-watering.jpg
 

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