Green pond water and other pond problems, and how to prevent them

Green water

The cause: microscopic algae floating in the water, fed by sunlight and nutrients. It's worst in spring and summer.

• UV clarifier. Water pumped past a UV light makes the algae clump together so the filter can catch it. Check the bulb is replaced yearly and the glass sleeve around it is clean.
• Shade. Cover a third to half of the surface with lilies or floating plants.
• Less food, fewer fish. Every bit of extra food becomes algae food.
• Don't drain the pond. A full drain and scrub removes the good bacteria, shocks the fish and the algae is back within weeks.

String algae (blanket weed)

The cause: long, stringy algae that grows on rocks, waterfalls and plants, usually in clear water with plenty of nutrients.

• Pull it out by hand or twist it onto a stick, little and often.
• Reduce nutrients: less feeding, regular partial water changes and keeping garden run-off out.
• More plants compete with it for the same nutrients.

Fish gasping at the surface

The cause: low oxygen, usually on hot nights, after a pump has stopped, or when there are too many fish. Oxygen is lowest just before sunrise.

• Run the pump and any fountain or air pump 24 hours a day.
• Add an air pump with air stones in hot weather.
• If fish are gasping, do a partial water change with conditioned water and add aeration straight away.

Predators

Grey herons are common in Cape Town gardens and can empty a pond of koi in a few visits. In some suburbs near rivers and wetlands, otters are a risk too.

• Pond netting or a low wire or fishing-line fence around the edge.
• Deeper sections and caves where fish can hide.
• Steep edges make it harder for herons to wade in.

Pump and filter failure

Leaves and debris block pump intakes, flow drops and the water quality follows. Check the pump intake weekly in autumn and after storms, and listen for a pump that sounds different. It's usually a sign of wear or a blockage.

Water level dropping too fast

Some water loss in summer is normal evaporation. If the level drops faster than a few centimetres a week in mild weather, or keeps dropping with the waterfall switched off, look for a leak in the liner, the waterfall or the pipework.

New concrete ponds

Fresh concrete releases lime into the water, which pushes the pH up to levels that harm fish. New concrete ponds need to be properly cured and sealed, and the water tested, before fish go in.

Garden chemicals

Insect sprays, weed killers and lawn fertiliser drifting or washing into the pond can kill fish quickly. Spray on still days, cover the pond if you're spraying nearby, and keep garden run-off away from the edges.

The pattern behind most pond problems

Almost every pond problem comes back to the same few things: a filter that's too small or neglected, too many fish or too much food, and no regular water testing. A regular service schedule keeps all three in check. See pond care through Cape Town's seasons for what to do when.