About Filmy Ferns
Filmy ferns occupy a number of genera, including but not limited to Hymenophyllum, Trichomanes, and Crepidomanes. The approximately 650 species are extremely varied in frond form, size, and growth habit. Some species are no larger than ¼” tall, and others have fronds much larger. The fronds may be entire, as a disc or fan, or intricately divided and feathery. Fronds come in a wide spectrum of colors, with most being shades of green, but some may have a glaucous appearance. Most have tissue which is only one cell thick, making it translucent to some degree, and earning them the “filmy” moniker.
Diminutive filmy ferns create fine carpets along horizontal or vertical surfaces, and large species create clumps and draperies on trees and rock faces.
Filmy ferns’ environment is usually very wet, often in cloud forests and fog prone locations, as well as rainforests with significant precipitation, which maintain the humidity levels and generally cooler temperatures necessary for their survival. They grow in areas where they are sheltered from dry air, such as pockets of moss, wet wood, and rock crevices. The plants may also be near splashing, dripping, or seeping water, which supplies constant moisture to their delicate fronds, and creates a protective barrier against air movement.
How to Grow Filmy Ferns
Most filmy ferns will require different growing conditions than the majority of other tropical plants which you may want to grow. Therefore, you’ll need specialized growing tanks solely for filmy ferns and other plants and bryophytes which do well in similar conditions.
With this in mind, let’s take a look at some culture tips for these delightful but challenging plants.
Humidity and Moisture
Filmy ferns require exceptionally high humidity, as would be found in their natural habitat. They have evolved to be adapted to these conditions, which is why their fronds are extremely thin (“filmy”) and have little to no cuticle:
“Hymenophyllaceae are a rare example of an evolutionary shift of adaptive strategy from typical vascular plant adaptation to the poikilohydry most typical of bryophytes.” 
In layman’s terms, the quote above means that, similar to mosses, filmy ferns have adapted to utilize their environment as way to maintain tissue hydration, as opposed to having structures or mechanisms for doing so within their foliage.
Since their leaves are so susceptible to drying out in the absence of moisture, you need to keep the humidity as high as possible. A Relative Humidity (RH) of at least 90% may be necessary to prevent desiccation, with higher RH being helpful.
Filmy ferns should be kept in extremely moist or wet conditions, which helps maintain high humidity and can also support tissue hydration by direct contact. Their rhizomes should never be allowed to dry out, and should be constantly in contact with, or covered by very moist or wet substrate. Many species benefit from frequent misting with pure water, which mimics their natural habitat.
Air movement should be avoided as much as possible. Even under humid and wet conditions, sometimes filmy ferns will dry out when exposed to air flow, even if it’s not directly blowing on the fronds. It’s best not to have fans in a filmy fern growing container, although in some cases you can pull it off if done thoughtfully and carefully.
When growing filmy ferns in a grow tank fans, you can use fiberglass window screen to reduce air flow over the fronds, as well as to hold in humidity and soften the lighting (see accompanying images).
If you must use fans, you can try using a digital timer with multiple on-off programs per day, so the fans are not on constantly.
When using fans, the ferns should be misted with distilled or RO water at least once a day to rehydrate any desiccation caused by the air movement.
As can be deduced by the humidity, moisture, and air flow sections of this article, filmy ferns should be kept in an enclosure which is entirely, or mostly sealed. Glass aquariums, jars, plastic tubs, and deli cups are good options.
Feel free to browse the Containers list on my Amazon page to see a variety of plastic bins and cups that work well for filmy ferns and other tropical plants.
Above is a photo of some grow bins on a grow rack, which work very well for growing filmy ferns. Check out my grow bin tutorial and my how to set up a grow rack series to learn more.
For watering and misting, use water that is very pure and low in TDS (Total Dissolved Solids), for example, distilled water or RO water. This mimics the purity of rain and mist from their natural habitat and avoids harming the delicate foliage.
The best substrate will depend on the natural habitat of the species, though in most cases sphagnum moss will be an excellent choice. As mentioned previously, the rhizomes should be covered by, or at least in constant contact with very wet substrate.
You can browse the substrates list on my Amazon page to see the Sphagnum that I use, as well as other substrates that I use for my plants and terrariums
Filmy ferns should not be planted in open spaces. They should be placed in sheltered locations, where wood, rocks, moss, and other surfaces will protect them from air movement and bright light. They will benefit from having their fronds close to, or partially enveloped by wet substrate and hardscape elements. Basically, the more sheltered you can make them, the happier they will be.
Do not plant filmy ferns high up in a tank, where they will be too close to light, heat, air movement, and other conditions that will harm them.
You may experiment with growing filmy ferns near water features, such as waterfalls and pond banks, where the rhizomes and fronds will receive constant moisture and humidity.
Filmy ferns can be grown on horizontal or vertical surfaces, though each species may have a preference. Keep in mind that vertical surfaces are more challenging to keep saturated.
Smaller species do well when partially enclosed in pockets and walls of sphagnum moss, which maintains high humidity in the direct vicinity of the fronds, and also may offer moisture to those which directly contact the wet substrate.
Some species of filmy ferns grow in areas where water is constantly flowing over the plant, or where the entire plant is seasonally flooded – you can experiment with growing them submerged in water. It’s important to use pure water, and to ensure that it stays fresh and does not stagnate.
If the species is suitable for growth under water, within a few weeks you should notice healthy new growth of rhizomes, and newly forming fronds. Aquatic leaves will usually be morphologically different from emersed leaves, and will die if taken out of water long enough to dry out completely.
Growing filmy ferns in water is a fun experiment, and when successful it results in beautiful and interesting growth. I’ve found that the growth rate under water is significantly faster than in terrestrial conditions.
Bright light should be avoided for filmy ferns.
“Light and desiccation responses of filmy ferns can be seen as an integrated package. At low light and windspeed in humid forests, net radiation and saturation deficit are low, and diffusion resistance high. Water loss is slow and can be supported by modest conduction from the sub-stratum.” 
The technical jargon quoted above can be translated loosely into layman’s terms to mean that lower light environments are more supportive of keeping filmy ferns hydrated.
Medium to low light, and indirect light, are ideal. Very shady spots are acceptable, but some species will grow substantially slower, or even stop growing if light is too dim.
By the way, I have a video that shows and discusses the lights I use for my plants, as well as a tutorial series for how to set up a grow rack.
Since filmy ferns grow in many different climates, there is not one global rule for temperature. Some prefer to be on the cool side, while others will do just fine in warm temperatures. If you know where a particular species came from, you can do some research to identify the ideal temperature ranges. When in doubt, intermediate temperatures are a good starting point. In general, you’ll want to avoid hot temperatures. In summery weather and hot locales, keep filmies in the coldest part of your home.
You can feed filmy ferns with liquid fertilizers at a very diluted strength, or a very gentle, slow-release pellet fertilizer, at a low dose. Use caution because some species may not fare well, and some fertilizers may do damage to the foliage. It’s best to start with extremely low doses and strengths, and test just a small portion of ferns. If they remain unharmed after several weeks, it should be OK to apply to the rest.
Some liquid fertilizers I have used are Grow More Urea-Free Orchid Fertilizer, and Sea Grow. They come as a powder which should be mixed with distilled water, to about 1/4 of the strength recommended on the packaging. Use a spray bottle to gently mist the plants every two or three weeks. A third fertilizer option is Osmocote Plus, a slow-release capsule. Use about 1 capsule per 4 square inches of fern clump. The capsules should release nutrients for 3-6 months.
See my plant fertilization video for more info, and pick up some of my recommended fertilizers on Amazon.
Companion Moss and Plants
Protect small and micro species of filmy ferns from rampant mosses and plants, which will choke them out. Larger filmy ferns should be OK with moss growth, but keep it in check so it doesn’t get too out of hand.