El Niño Fern from Pestmart? (I should know better by now)

There's a very good chance that it is just not an aquatic plant. If you could post a pic of even do an image search and find a match that we could see... perhaps it's a java fern and the browning could be normal. I have some do that as they produce daughter plants.

In any case the ferns typically are a rhizome plant and if you bury the rhizome (roots) they can rot and the plant will die off. Usually these are attached to driftwood or rocks but large gravel substrate might not be as bad as smaller grained gravel or sand.

Jeff.

You know, you could just float them on a piece of wood weighed down a bit to partially submerge it or tied off to a rock on the bottom with some fishing line.

Jeff.

Yah, I realized that you were going to do a cave after your cave shot... I missed that in the other posts. Floating hunks of wood don't offer as much in hidey spots.

Jeff.