By selecting the right aquatic plants and the right combination of plants you will never have to use filters in your garden pond. Plants act like a natural filtering system. Initially, the water will turn green as the free-flowing algae increase and absorb mineral nutrients, but, when the aquatic plants become well established, the algae soon disappear and the water will become clear and translucent. It can take a few months for this water ecosystem to reach the right balance.
Aquatic plants like water lilies and floating hearts have leaves with large surface areas that cause shade under the water surface which discourages algae. Other aquatic plants absorb minerals from the water which causes algae to die resulting in clear water. Sometimes it takes a while but is well worth the wait.
Fish are also an important component. Their gills absorb oxygen secreted by the plants and, in turn, the plants absorb carbon dioxide that is secreted by the fish. The fish residues and nitrate excretion serve as food for the plants. It gets broken down by bacteria and absorbed by plants. The end result is a healthy pond in balance.
Plant the aquatic plants in sandy to gravelly soil and mix one-third compost with two-thirds of gravel. Cover the top with a layer of gravel of about 1 cm thickness which will prevent the compost particles from floating in the pond.
When it comes to aquatic plants we mainly get three groups: underwater (submerged) plants, free-floating and riparian species. Underwater plants can again be divided into species with floating leaves and the underwater types.
Plants with all their organs underwater.
These plants fulfil an important function in water purification. They absorb nitrates from the water, causing free-floating algae to disappear. They also provide oxygen to fish. Some of these plants can also grow so dense that it can clog a pond. Watergrass (Vallisneria aethiopica) looks like green grass and grows and spreads like a lawn underwater. This species has a worldwide distribution and there are several cultivars available including some with spiral leaves. Free-ranging species with climbing stems include the fine Oxygen grass (Lagarosiphon muscoides) (also known by the name Elodia crispa) and Coontail (Ceratophyllum demersum).
Both increase rapidly and also provide shelter for fish and turtles. The aquatic plants with floating leaves have an underwater growth point and include the native Water clover (Marsilea schelpeana), Floating hearts (Nymphoides indica) with its beautiful yellow flowers, and the Water lilies (Nymphaeae nouchali var. caerulea of which I recommend the Cape water lily (Nymphaea capensis) with its blue flowers and yellow center. They all grow easily and provide shelter to tadpoles and fish. The Waterblommetjie (Aponogeton distachyos) with its white flowers is just as attractive and an asset to any garden, both the water lily (Nymphaea capensis) and Floating hearts (Nymphoides indica) multiply rapidly from seed, and the Floating hearts can be grown from leaves.
As for the riparian species, they range from creepers to trees. Be careful of planting trees or shrubs too close to the dam as the roots can cause damage. The local Mat sedge (Cyperus textilis) is an attractive species and then there is the Miniature papyrus (Cyperus prolifer) which grows up to about 60 cm. Bulrush (Typha latifolia) is fast-growing and should preferably be planted in larger water gardens as they can take over quickly. The same goes for the Common reed (Phragmites communis). Both are in informal gardens and will quickly attract birds to the garden. The River star (Gomphostigma virgatum) has white flowers and is a shrub reaching up to 2 m high. It is especially suitable for domestic gardens.
The Vleiblommetjie (Onixotis triquetra) is a bulb with a variety of white flowers. The River bells (Phygelius capensis) are a herbaceous riparian plant up to 1000 mm high with striking red tubular flowers that attract sunbirds. The Wild mint (Mentha longifolia subsp. capensis) is a sharp-scented plant with fine white to rose-coloured flowers. The Water crinum (Crinum campanulatum) has beautiful pink bell-shaped flowers and is native to Eastern Cape. It often grows in water and can be planted in a container inside the fish pond. Other Crinum species are also good on the banks.
The Arum lily (Zantedeschia aethiopica) is also an excellent choice. It forms beautiful bulbs on the banks with their striking large white flowers during the spring. It is found even in shallow water and swampy areas. There are also many Kniphofia that like wet conditions and are suitable for water gardens.
For local Cape Fynbos gardens, I recommend the beautiful Horsetail restio (Elegia capensis) with its soft leaves and the Cape thatching reed (Elegia tectorum). The Palmiet (Prionium serratum) grows large and is suitable for large water gardens). Schoenoplectus littoralis is a large varietal that is also suitable.