I'm not an Adiantum specialist, but that looks like "water stress" to me. Maidenhairs are fussy about water. They do best with perfect drainage and freqent watering with good water. (not in standing water) Your water quality and humidity look fine. Contrary to advertizing they can be tricky to grow indoors. Even, difficult to establish outdoors.

A. capillus-veneris (maidenhair or venus hair fern) is native here in the SE region of Colorado so zone woud be about the same. Below is a nice picture of the species. Here is a link that might be useful: A. capillus-veneris

In my zone 8b garden in Alabama, I have my maidenhair ferns in the ground. They die back but come out again early in the spring. I have mine in a full shade location, in a protected area behind a tall privacy fence. I always mulch with pine straw or loose leaves and pull that cover back in early spring. I'm not familiar with the zone for Pac NoWest, but if it is anything like Seattle, you are not in the deep freeze winter area. If you choose to keep your fern in a container, be sure to surround the POT with protection for the root system, and then loosely cover the planted area as well. Some ferns like dryness, instead of wetness, and I don't water my ferns in the winter...unless they are indoors. Root rot is a big problem, as you probably know.

All of the cultivars you listed are variants of the species Adiantum raddianum(Delta or Pacific Maidenhair). Hardy only to Zone 9(20°F), they are grown as HP in most of the US during the winter and moved outdoors after last chance of freeze in the spring. They can be maintained indoors, as Maidenhair Ferns do not require bright light, they do, however, require evenly moist soil and moderate humidity. Forced air heating, which lowers the indoor humidity, unless a humidifier is installed, can cause failure of the plant, unless certain precautions are taken. One of the easiest ways to provide humidity, is to use a 1-2" deep medium to large tray under the plant. Place pebbles in the tray and set the pot on them. Keep the tray filled with water to a level just below the bottom of the pot. Provide some air circulation to help prevent pest and diseases from attacking it. The Southern Maidenhair is native next door to you, in SW SD (Hardiness Zone Unknown, but could be Z5a). It is also a typical Maidenhair, requiring even moisture for the roots, and goes dormant in the winter.It could possibly be grown as a houseplant, but I have no experience growing them indoors. Use caution when dealing with ebay vendors. Check all comments for problems. Personally, I do not purchase anything there or even load the website anymore, as it always downloaded a lot of Spyware. Rb Here is a link that might be useful: Adiantum Care Notes