Microsorum pteropus narrow is a slight variant from the "normal" Java Fern. It has very narrow leafs that would produce a grassy appearance in a large aquarium.
Microsorum pteropus should be grown on a root or stone, attached with fishing line until it has gained a hold. If it is planted in the bottom, do not cover the rhizome because it will rot. A hardy plant which grows in all conditions. The black spots under the leaves are sporangia (reproductive organs), not signs of disease as many believe.