Microsorum pteropus, commonly called Java Fern. There have been several varieties cultivated by nurseries. From the photo it seems to be attached to something, a chunk of wood (?), which is good, as burying the rhizome can cause it to rot and kill the plant. The rhizome is the thick stem-like part, from which fronds (= leaves) and fine black roots grow. The plant assimilates nutrients from the water via its leaves and roots.
Like most ferns, it is slow growing, so light can be low/moderate and fewer nutrients are needed. In too bright a light the fronds (leaves) will begin to turn transparent. Does best under some floating plants so it is not in direct overhead light. most fish tanks will have sufficient nutrients, but that depends upon the number of plants, and other plant species that may be faster at using the nutrients. A comprehensive liquid fertilizer is probably a good idea; Flourish Comprehensive Supplement for the Planted Aquarium is what I use, there is also Brightwell Aquatics' FlorinMulti.
Once established, Java Fern reproduces by adventitious plants at the tips of the fronds (leaves). The plantlets may be pulled away from the frond when they have a few fronds and roots; if left, the frond tip will eventually darken and the daughter plant will break away and drift until the roots are able to attach themselves to an object. The plant may also be propagated by cutting the rhizome (ensuring there are a few fronds and roots attached to each piece) and affixing it elsewhere.