If you manage to achieve the right balance of plant life and animal life, then your pond should remain relatively clear without any further assistance. However, from time to time you may have problems with algal growth.

Algae

The growth of algae depends on sunlight, carbon dioxide, and dissolved mineral salts. Introducing plants with broad surface leaves (deep water plants and surface floaters) will reduce the amount of sunlight the pond receives. Aim to cover about a half to two-thirds of the water's surface with plants. Oxygenating plants will minimise the amount of carbon dioxide in the water, which will effectively starve the algae. Clearing out dead leaves and other decaying matter will decrease the amount of organic matter that the algae has to feed on.

There is also a filamentous form of algae called blanketweed (Spirogyra) which may be found in ponds with otherwise clear water. This weed will restrict the movement of fish and choke other plants, so you must take care to remove it every so often. You can do this by winding it onto a stick or lifting it out with a garden rake.

Other Maintenance Tasks

Keep the pond clear of decaying plant matter and fallen leaves as they will reduce the amount of oxygen in the pond and encourage the growth of algae. If the pond is positioned near deciduous trees and shrubs, you may want to consider covering the surface of the pond with a net in autumn until all the leaves have fallen.

Thin any overgrown plants every so often; either pull out a few handfuls or lift the container and cut back to one-third. Do not thin all the plants at once as this will affect the water balance drastically and encourage algal growth.

You will need to prevent the pond from forming a solid layer of ice during the winter; this will trap poisonous methane gas released by decaying vegetation and could kill any fish you might have. Before winter sets in, float an inflated ball on the surface of the water to keep the pond from freezing over entirely. When ice forms, pour boiling water over the ball to remove it and then cover the hole with a piece of sacking. If the weather becomes milder and the ice begins to thaw, remove the sacking and replace the ball.