No no no!! Java ferns MUST be put on decor!
Java ferns (Microsorum pteropus) can be either planted or put on decor to be honest, I've done both ways and grows absolutely fine, quite a pretty plant and a nice addition of a easy to care for plant imho.
Just do not plant or cover the rhizome of the java fern into the substrate is all.
Any substrate should be fine, gravel or sand etc
Grows to about 15cm tall, addition of liquid ferts will be beneficial but not essential.
Sometimes you might see black/brown spots on the leaves after a while, this is not a sign of disease as some might assume, the spots are part of the normal growth as is sporangia (hormone or reproductive stages I think if remember right)