The Java Fern doesn't like having its rhizome buried in the substrate and usually does better when it is tied to an object like driftwood or an ornament. You can use string, fishing line, super glue or a rubber band to hold the plant's rhizome on the ornament.
Other good plants to try include Hygrophila polysperma, narrow Vallis and Water Sprite (Ceratopteris thalictroides).
Water Sprite is a floating plant that can also be planted in the substrate where it grows into a lovely light green shrub. The other plants should be grown in the gravel.