Pond plants and oxygen .. oxygenators best

OXYGENATORS .... THE ESSENTIAL INGREDIENT
A pool or a pond in a natural wild state is a self-sustaining little world. As in our bigger world, the inhabitants need oxygen to survive. This can be provided by plants growing in that world. If you want to have a naturally balanced pool that requires the minimum of fuss and
maintenance, without having to oxygenate the water mechanically, then plants that release oxygen into the water (i.e. oxygenators) are the essential ingredient.

BIT OF BIOLOGY
All plants, apart from parasitic ones, from single celled algae to Baobab trees, produce oxygen during a process called photosynthesis. This is a reaction in the leaves (if it has them) by the green chloroplasts of the plant cells, that uses sunlight or ultra violet light and carbon to create carbohydrates and sugars used as the building blocks of the plant. The carbon used is from carbon dioxide,
which is taken from the immediate environment. One of the by-products given off in the process is the oxygen from bond of CO2. This two-way exchange of gases is called transpiration.The oxygen is lapped up by the animal inhabitants great and small of that environment. But in a pool or pond, it is just as gratefully received
by the (aerobic) bacteria in the bottom of the pond, or perhaps in your case a filtration system, to aid them in the process of breaking down organic matter to its constituent chemical parts. It is important to consider some of the compounds (like nitrates) produced by the
bacterial action, will be used by the plants in the pond environment to boost the growth activated by the photosynthesis. And if there are not oxygenators and other higher plants there to use them up, then algae will. The result will then be green water as the algae proliferate. You see oxygenators are essential, unless you have a fountain, waterfall, air block or venturi to do the necessary instead.
BUT.... It must be pointed out that oxygenators do not work a 24 hour day. 'When the lights go out', oxygen ceases to be produced and plants join the queue for the demand for oxygen. So in a very heavily stocked pool that is overloaded with underwater plant life, resources can be somewhat stretched.

ON THE OTHER HAND... If you keep within the certain prescribed limits then everything should swing along nicely. Allow yourself only 2ins of fish per square of pool surface area (or 50cm for every square metre) in a pool established with 2 bunches for every square foot
(20 for every square metre)... that is unless you keep KOI carp.In the latter circumstance, unless you want to give your fish a good feed, keep them well apart. In certain Koi pools in the past I have found it convenient to have a planting of oxygenators in the header pool to a stream, which seemed very effective. They work like
filter brushes, slowing down the flow of sediment that even manages to bypass the filter system. The only problem was that they needed to be regularly replenished when the header pool had to be cleaned out, (no problem, see article replanting oxygenators).

THE GOOD AND THE BAD... Oxygen dissolves from oxygenators into the water more effectively than by mechanical turbulence because it is in fact very difficult to dissolve oxygen into water.
It needs to be done slowly and over a large surface area.
All plants that live happily under water can be described
as oxygenators. Out of these plants there are bound to be one or two that are the best for the job we need them to do. There is no doubt they are specialised. Roots tend to be merely for anchorage and the nutrition absorption and gaseous exchange occurs on the surface of the plant directly to each cell.
The plant therefore has very thin walls and thin leaves to allow this to happen. This makes the plants floppy, which in fact becomes an advantage under water as they are able to bend with the eddies in the water. Two plants that have made this a real speciality and thrive in streams are Water Crowfoot, the true Water Buttercup (Ranunculus aquatalis) and Curly Pond Weed (Potamageton crispus).
THE BEST by far for the pool or pond is also often referred to as "Curly Pond Weed" but is most rightly called Laragasiphon major or more commonly Elodea crispa( South African native).
This has a tendency to take advantage of highly nutritious pond environments by filling them up, but at least you don't get algae.
It is easy control by just snapping handfuls of it near to the source of growth. Fish love to spawn on or near it and it is dense enough to provide protection for the eggs and the subsequent fry.
For conservation and wildlife gardeners it is not entirely kosher since it is not a native plant.
For indigenous plants to the UK: Water Starwort, (Callatriche stagnalis), rampant but easy to crop to its source.
Water Milfoil, (Myriophyllum spicatum), loves limey pools with high pH and does well where Elodea crispa fails. You get the bonus of little flowers in some years that looks magical in certain lights.
Hornwort, (Ceratophyllum demersum), this is often confused with Milfoil out of the water, but it feels much stiffer and has more forked growth habit. This is one that does not mind a bit of shade on the pool.
Water Violet (Hottonia palustris), class act for the connoisseur; it has pretty little pink flowers above the water surface in May and June.

THE WORST....... The worst thing about some oxygenating plants is not that they are useless at their job, but that they are very often confused with the best, or they are sold as marginal plants and quickly run rampant particularly in clay-lined ponds and then are
impossible to eradicate.
Canadian pondweed (Elodea Canadensis), also known as Anacharis, can easily be confused with the weaker growth of Laragasiphon major.
It will even grow on the surface of marginal plant baskets.
Parrots Feather, (Myriophyllum proserpinacoides) this is causing some consternation in the States in areas where the frosts are not stiff enough to knock it back. Sold as a marginal here, it has submerged and surface foliage and will leap from one side of a small to another in less than a season.
Mares tail (Hippurus vulgaris), often sold as a marginal. This plant has been around since time began, so it is not without a trick or two up its stem!
 

This page by courtesy of Peter J May, the Perfect Pond Detective