Indian fern is Ceratopteris pteroides and also known as water sprite (which confuses it with C. thalictroides) and water wisteria, Hygrophila difformis. There is a poor pic of it here;
http://www.wetwebmedia.com/PlantedTksSu ... pteris.htm
If you have a dense clump of Java moss it will sink but thinned out, it floats. The India fern has long floating roots for the fry to hide in.