Gsm having some issues.

Grown in build a soil , she looks like shes hungry for potassium but i cant figure out why the leaves are curling downward all other plants in the tent are fine. I can deal with potassium deficiency but im stumped as to why they are curling down does this look like anything obvious to anyone? These have been water only with build a soil havent had an issue in over a year that i couldnt nail down the cause but this one has me stumped. Maybe the beginning of root rot?