Salvinia minima – Water Spangles is actually a floating fern whose 3/8″ long fuzzy leaves (covered with white hairs) or fronds float on the the pond surface. It helps to absorb excess nutrients in the pond and shade out water-borne algae, and provides welcome cover for fish. Considered an invasive plant where hardy (Texas, North Carolina). Hardy to zone 9.
Ceratophyllum demersum – Hornwort is a common oxygenator usually sold in bunches. The submerged stems (which do not root) absorb CO2 and release oxygen as they photosynthesise. A good choice for deep and partially shaded ponds, fish will also readily spawn on it. Overwinters as stems that sink to the bottom and produce new foliage in spring. Hardy to zone 5.
Myriophyllum aquaticum – Hardy Parrot’s Feather serves as both a surface and sub-surface oxygenator, as it billows over the water once it emerges. The feathery green foliage is a heavy feeder (good for absorbing excess nutrients) but should only be used in closed pond systems so it doesn’t become an invasive. Stems below the water surface survive winter. Zone 5.
Lemna minor – Duckweed is a common sight in local ditches and lakes but it is also an effective floater for ponds. The light green leaves will quickly cover the surface (can just be netted out when it gets excessive) but may be lost in skimmers. Pond fish love to eat this plant, so many people grow their Duckweed in large containers and add it from time to time. Hardy to zone 4.
Hydrocharis morsus-ranae – Frogbit has small 1″ leaves that emerge with a bronze tint and mature to a glossy green, these much resemble a miniature waterlily. It bears small (3/4″) white papery flowers with pale yellow bases in summer. A native of Europe, North Africa and parts of Asia, it overwinters on the pond bottom as a dormant bud. Hardy to zone 6.