Süsswassertang (German spelling: Süßwassertang; see ß) is a type of aquarium plant. It was long considered to be a liverwort, which it strongly resembles, but in 2009, a molecular phylogenetic study determined that it is, in fact, a fern gametophyte. Further, it is a species of Lomariopsis. It is closest to Lomariopsis lineata, but may be a new, unnamed species. Many reference sources on the web describe it as L. lineata, but its inclusion in that species has not been validly determined. Efforts to induce the plant to form a sporophyte have failed, which may indicate status as a new species. This plant was first mis-identified as Pellia endiviifolia before the analysis that determined its true status.
The name Süsswassertang means "freshwater seaweed" in German. It is commonly incorrectly spelled subwassertang due to the German eszett's (ß) similarity to the latin B. It was formerly known as "round Pellia" or "round-leaf Pellia".
Reproduction is by fragmentation. Pieces that break off develop into new plants. The plant is thalloid in form, and exhibits indeterminate growth. It is profusely branched, and 1 cell-layer thick. Because of its lack of morphological characters, it has been difficult to determine its systematic position. Gametangia are rarely produced, but they are clearly fern-like. The archegonia have short necks and the venter (base) is partly sunk into the thallus. The antheridia are like those of polypodialean ferns in that they consist of a basal cell, a ring cell, and a cap cell.