Use the free Pond Algae Control Plan below, right in your browser. No sign-up, no software to install, and nothing is sent to a server.
Green water and blanket weed are the most common pond complaints, and both come from the same recipe: sunlight plus nutrients plus still water. Algae are plants, and they grow wherever light and food meet. This planner scores your algae pressure from pond size, sun hours and fish load, then builds a plan around shade, plants and water changes that removes the causes instead of just treating the symptoms.
Enter your pond volume in litres, the hours of direct sun the pond receives each day, and your fish load. The tool scores sun at eight points per hour, adds fifteen points for medium stocking and thirty for heavy stocking, and adds ten points for small ponds under one thousand litres, which warm and swing faster. Scores below forty are low pressure, forty to seventy are moderate, and above seventy are high.
Sunlight is the lever most pond owners underestimate. A pond in full sun for eight hours a day receives far more light energy than the same pond shaded for half the day, and algae use every bit of it. Shading forty to sixty percent of the surface is the single most effective algae step: floating plants, water lilies, a pergola or a shade sail all work. Aim for dappled shade, not total darkness, because submerged plants and fish still need light.
Plants compete with algae for the same nutrients, and a well planted pond starves algae quietly all season. Submerged oxygenators such as hornwort absorb nutrients directly from the water, while marginal plants around the edge take up runoff before it reaches the pond. Plant generously in spring, let plants establish before judging results, and thin them in summer so they do not choke the pond themselves.
Fish load and feeding set the nutrient side of the equation. Every gram of food becomes fish waste or decays into nutrients, and heavy feeding in a sunny pond is almost a guarantee of green water. Feed only what fish eat in two minutes, skip feeding on very hot days when oxygen is low, and do not add fish to fix algae, because more fish means more waste. If your score is high mainly because of stocking, the honest fix is fewer fish or a bigger filter.
Water changes dilute nutrients and reset the pond, but they must be done correctly. Change ten to fifteen percent weekly rather than a huge change rarely, because large changes swing temperature and chemistry and stress fish. Always dechlorinate tap water, match the temperature roughly, and pour the new water in slowly. A steady small change beats a dramatic large change every time.
UV clarifiers help with green water specifically, because they kill the single-celled algae that cloud the water as it passes the lamp. They do not kill blanket weed, which grows attached to surfaces, and they do not remove nutrients, so green water often returns when the lamp ages. If your water stays green despite shade and plants, check the lamp age, the flow rate through the unit, and whether the quartz sleeve is clean.
Blanket weed needs a different approach from green water. It grows in shallow sunny margins and on the liner, and it thrives where nutrients and light meet a surface. Pull it out by hand or with a stick regularly, improve circulation in shallow bays, and shade those margins with marginal plants. Barley straw and commercial treatments can help, but they work slowly and they work best after the physical removal and shading are already in place.
Runoff is a hidden nutrient source that many plans miss. Fertiliser from lawns, soil from beds and debris from trees all wash into the pond after rain and feed algae for weeks. Keep a raised edge or a gravel strip between beds and pond, do not fertilise right up to the water, and net the pond in autumn. A pond that receives clean rainwater stays clearer than a pond that receives garden runoff, whatever else you do.
Judge your plan over a season, not a week. Algae respond to weather, and a hot sunny spell can green a pond that was clear in May. Track your score, keep the shade and plants in place, stay steady with weekly changes, and resist the urge to reach for algaecide at the first green tinge. Algaecides can crash oxygen when large amounts of algae die at once, which harms fish more than the algae did. Steady habits clear ponds; quick fixes usually do not.
Versión en español: Plan contra algas en estanques – Sombra, plantas y cambios