Browning edges on some of my pants

Howdy. Long time listener first time caller.

A few of my plants have brown edges or spots.its only on some of the plants in the the section and I've quarantined the healthy plants and any new arrivals.I've read different things that it could be but need help nailing it down. The details
Currently the end of winter
West facing with decent sun from the window difused by sheer curtain
Watered to schedule based on pot size and species
Various different styles but all have similar problem

Any help would be appreciated