Clear water is healthy water, not sterile water

A little algae in a garden pond is a sign of life, not neglect. The trouble starts when the water turns the colour of pea soup, or long ropes of blanketweed smother everything in sight. Both are symptoms rather than causes: too many nutrients, mainly nitrogen and phosphate, combined with plenty of light. Off-the-shelf chemical treatments promise a quick fix, but they usually kill the algae, leave the nutrients behind, and knock back the water fleas, snails and amphibian larvae that make a pond worth having in the first place. Deal with the cause and the water clears largely on its own.

Plant generously, and plant native

Plants are the most patient nutrient removers you will ever own, and they do it for free. Aim for roughly half to two-thirds of the surface shaded by floating leaves at midsummer, without covering the whole pond — leave some open water for dragonflies and for birds to bathe.

• Oxygenators such as hornwort, spiked water milfoil and water starwort mop up dissolved nutrients and shelter tadpoles. Roughly one bunch per square metre, weighted or in baskets.
• Marginals including marsh marigold, water forget-me-not, water mint and yellow flag iris draw nutrients through their roots and give emerging damselflies somewhere to climb.
• Deep-water plants like native water lilies shade the pond and cool it, which slows algal growth.

Please check before you buy. Floating pennywort, water fern, parrot's feather and New Zealand pygmyweed are invasive and must never be planted outdoors or tipped into a watercourse. A good nursery will label natives clearly.

Barley straw, used properly

Barley straw is a long-standing British favourite and it does work, but only if you treat it as prevention rather than a cure. Put it in during spring, once the water has reached about 10°C and before the algae gets going. The usual rate is around 25g per square metre of surface area — a loose handful for each square metre. Loosely pack it into a mesh bag so water can flow through, and float it near the surface or place it where there is some movement. It needs oxygen to break down into the compounds that inhibit algae, so a still, stagnant corner is the worst place for it. It lasts four to six months; lift it out and compost it in autumn.

Beneficial bacteria and the sludge layer

Sludge on the bottom is not just dirt. It is leaf litter, fish waste and dead plant matter, and the bacteria that digest it release nutrients slowly instead of in one summer surge. You can support that process with a proprietary sludge-digesting bacteria product, but only when the water is above 10°C and well oxygenated; in cold, still water the bacteria simply will not establish. This is where a small air pump, fountain or solar aerator earns its keep — oxygen improves everything else in the pond, from fish health to the breakdown of waste.

Resist the urge to strip the pond clean. Sludge is habitat for overwintering insects and newts. If it builds up badly, remove a third of it in late summer or early autumn, leave the rest, and repeat over several years rather than all at once.

Feed less, and mind what else washes in

In most small ponds, uneaten fish food is the single largest nutrient input. Feed only what the fish clear in five minutes, once a day in warm weather, and stop entirely once the water drops below 10°C. Be honest about stocking levels, too — a small pond suits a few goldfish or none at all, and a fish-free pond will always support more frogs, newts and invertebrates.

Top up with rainwater from a butt rather than tap water, which brings phosphate, nitrate and chlorine with it. Keep lawn fertiliser, weedkiller and paving runoff out of the pond by leaving a gravel or planted margin between grass and water. And avoid copper-based algaecides and blue pond dyes altogether; they are indiscriminate and do real harm to the wildlife the pond exists to attract.

A simple year-round rhythm

• Spring: barley straw in, bacteria dosed once the water warms, overgrown plants divided and re-potted.
• Summer: twirl blanketweed onto a stick and remove some, but leave a little for cover; top up with rainwater.
• Autumn: net out fallen leaves before they sink, and leave one corner of marginal growth uncut for shelter.
• Winter: stop feeding, do not stir the sludge, and float a ball in the ice to keep a breathing hole open.

Do those small things consistently and you will get the result you were after: water clear enough to see the newts, without a single chemical in sight.