Keeping the pond clean
Dead and dying plant material rots, polluting the water, so use a fine mesh net to clean out the pond each week from spring to autumn.
If trees are near your pond, suspend a net over the top of the pond to catch any falling leaves - particularly in October.

Don't feed any fish unless the water temperature exceeds 10°C (50°F). You will notice when they are active and searching for food - over winter they live on fat reserves and plant roots. Feed them sparingly at first so that any uneaten rotting food will not pollute the water.

Maintain about 60% surface cover by planting water lilies. It helps to keep the water temperature low and restrict algae growth. There will still be an annual spring flush of green water when temperatures warm up, but your pond should clear when the lilies start to grow.

In autumn, when marginal plants die back around the edge of the pond, cut off the surplus foliage before it drops into the water.
Clean out your filter and pumps (if fitted) in autumn and at other times in accordance with the manufacturer's instructions.
Tip: Leave the net in place over winter if herons are a problem.

Keeping the pond in balance
Keep an eye on the pond in warm weather and replace evaporation losses regularly, but gradually, to prevent a sudden influx of fresh water.