please help save my plant!!

can somebody please telll me what this is and how i can fix it?? it started at the bottom and is spreading upward pretty fast. the weird thing is itts only happening to one of the plants. the other one has slight drooping and seems to have grown out of the light twisted leaves and is doing great now. just recently flushed the problem pant with 6.5ish ph water to try and help if its a lockout due to high ph? they both have been watered with 7 ph tap water using the tube test indicator drops. im still watering the healthy looking plant with 7ph tap because it doesnt seem to be negatively affecting it at all.











 
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