Here are some more detailed shots to show how I potted these plants up.
For three plants I used planters filled mostly with Hydroton, and with a cap of finer gravel.
For two of these I also included a layer of marine crushed seashell substrate. Many of the references on Adiantum
that I encountered indicated that they grow best in "sweet" (alkaline, high pH) soils, and that they benefit from the addition of powdered lime to potting medium. I figured that the crushed shells could act as a slower-release source of hardness minerals for the root environment in the pot.
While I have not seen any mentions of Adiantum
actually growing as aquatic marginals, these ferns do apparently have an affinity for permanently saturated spots on rock faces or the edges of small streams. Have a look at the habitat in the final picture of this linked page, described as "a continually wet limestone face covered with maidenhair fern"...
While the plant roots will be below the water's surface, rather than growing along the face of wet stone, I hope that this condition will be close enough to the kinds of natural habitats that many maidenhair fern species prefer.
I planted the last of the four divisions in a planter mostly-filled with the finer planter gravel. I also added the crushed shells to that container, along with a quantity of the clay powder + MTS amendment that I like to use.
Here's one more picture.