El Niño Fern should not be planted with its rhizome (roots) buried in the substrate because it will rot and recede. Instead, it can be attached to rocks, driftwood, and other decorations in the aquarium, or it can be planted on top of the substrate as long as the roots are not completely buried.
Here we go again. I spent several years warning my readers in my plant column to be aware of plants being misrepresented and relabeled with trendy names purely for marketing purposes. I also contacted the 'big box' suppliers asking them to stop misleading the consumer. From what I can gather, your 'El Nino' plant is most likely a Bolbitis. It is a slow growing plant best suited for aquariums. Will you please post an image of your plant?
Wow, Top Fin should get credit for providing the binomial nomenclature and the plant's intended use. That's very honest of them. Notice on the container is says, 'For aquariums.' Your plant can grow completely submerged or emersed, which is most likely how it was propagated. But, it was clearly sold as an indoor aquarium plant. From the grower to the retail center, it has been contained a very humid, protective container under artificial lighting. Suddenly taking it outdoors and planting with the leaves fully exposed to sunlight is most likely why your plant is drying out. Without a gentle transition the results are the plant becoming sunburned and dehydrated. I would find a spot that is fully shaded all day and if possible cover it for awhile with an opaque container to hold in humidity and reduce being blasted by reflected sunlight. You can also completely submerge the plant and allow it time to acclimate.