Foliage of Fern Turning Brown
I have a giant fern I bought in California 34 years ago. I have grown it as an indoor houseplant since moving to Illinois in 1975. Several years ago, it developed a dark rust-like browning to the ends of the fronds. I have transplanted it several times since owning it, but not in the past 5 years. We also changed our water system over to a softener but I try and give it well water most of the time. It sends up new fronds only to have the leaves turn brown from the edges inward.
You have done some great work in evaluating potential causes of the problem. Continue the process by carefully sliding the plant out of its container. Although ferns like to be a bit potbound, growing in overly cramped quarters can result in stunted growth and the symptoms you described. Move the plant to a slightly larger container if needed. Be sure to loosen any circling roots before replanting. Salt used in water softeners can also cause browning and death of plant parts. Continue to use well water or collect rain water, melted snow or the water from your dehumidifier. These will eliminate the risk of salt burn. And lastly make sure the light conditions have not changed. Often as trees mature, awnings are installed or other features are added outdoors they can impact the light reaching our houseplants. Though most ferns are shade tolerant outdoors they prefer a brightly lit location inside our homes.