Fish ponds: build, fertilise and harvest an earthen pond

Fence it, and keep a rescue pole on the bank. Drowning kills about 300,000 people a year; children under 5 are nearly a quarter of them, and 92 % of the deaths are in low- and middle-income countries (WHO, 2026). WHO's first measure is to control access to water hazards with barriers.

• Fence every side the houses and paths face, with a gate kept shut.
• Rescue pole or throw rope hung at the bank: rescuers reach or throw from the dike instead of wading in.
• Supervised childcare for children under school age while adults work the ponds.
• Teach school-age children basic swimming, water safety and safe rescue (WHO).

Gravity in, gravity out. Look for ground sloping 0.5–3 %; avoid slopes over 5 %, and expect flat land to make drainable ponds dearer (FAO, Pond construction). Plan 2–5 l/s of water per hectare of pond, by gravity if at all possible. Read the flood marks on trees and bridges and ask how high exceptional floods reach. Take the water in upstream of fields sprayed with pesticides, which FAO names for cotton, citrus, tomatoes and rice.

Test the soil at every depth the pond will cut to (FAO, Soil):

• Squeeze test: wet a handful until it sticks together but not to your hand and squeeze hard. If it keeps the shape of your hand, there is enough clay.
• Bottle test: 5 cm of soil in a bottle of water, stirred, left for an hour: sand settles at the bottom, silt in the middle, clay on top. Silt and clay together should be more than 50 %.
• Avoid rock outcrops, gravel beds, sandstone, limestone, peat, termite mounds and the roots of big trees. Sandy clay, silty clay loam and clay loam are best.

Start with one pond of about 400 m² (20 × 20 m). Give the bottom a fall of about 1 % from inlet to outlet, 20 cm over 20 m, so it drains completely: about 0.8 m of water at the inlet and 1.0 m at the outlet. Keep every part at least 0.5 m deep, or water plants take hold; deeper than needed costs dike earth fast (FAO). Run the longest dikes parallel to the prevailing wind, which mixes and aerates the water.

Dikes for a 400–600 m² pond, from FAO's Table 27 (good clay soil; fair, loamy soil in brackets):

• Water 1.0 m, freeboard 0.30 m: design height 1.30 m.
• Crest 1.0 m wide (1.3 m), never narrower than the water is deep.
• Inner slope 2:1 (2.5:1), outer slope 1.5:1 (2:1).
• Build 15 % extra for settlement: 1.53 m high, on a base 6.4 m wide (8.2 m).

The crest is level all round. The drawing above gives the dike dimensions; the drainable fish pond build takes it through construction, and Irrigation and water control shows the water level and the channel.

Strip the topsoil from the dike and pond area and keep it; take all levels from the base of the topsoil. Where the ground under the dike lets water through, dig a cut-off trench along the dike's centre line, about 0.5 m wide and 0.6–1 m deep for a small dike, and fill it with rammed clay. In sandy soil, build the dike around a clay core (FAO).

Lay the earth in horizontal layers 15–20 cm thick, each wetted until a handful forms a firm ball, and ram each before the next. A hand rammer is a 4–6 kg weight of about 150 cm² on a wooden handle. Ram the finished slopes smooth.

Grass it at once: spread 10–15 cm of the saved topsoil on the crest, the outer slope and the inner slope down to the water line, and plant a creeping perennial grass. Never plant trees on or near a dike; their roots open leaks.

Inlet at the shallow end, outlet opposite (FAO).

• Inlet: a pipe or gutter with its floor at least 10 cm above top water level, reaching 0.6–1 m out over the pond so the water splashes in. Screen it: wild fish eat the food and the fry.
• Fill rate: a 400 m² pond averaging 0.9 m deep holds about 360 m³; at 7 l/s it fills in about 14 hours.
• Outlet: a monk, a three-sided tower closed with boards and a screen in grooves, draining through a pipe under the dike. It sets the level, acts as the overflow and empties the pond. For a pond of 200–400 m², FAO sizes the pipe at 10–15 cm inside (never under 10 cm behind a monk), and the tower 30 cm wide inside. A plugged pipe or a siphon serves the smallest pond.
• Catch basin: a hollow at least 30 cm deep in front of the outlet, where the fish gather as the pond drains.

Measure the pond water at noon through the year and choose fish that grow in it. Best ranges for adults (FAO):

• Nile tilapia: 27–30 °C; in danger below 12 °C. Eats phytoplankton, so it thrives in manured green water.
• Common carp: 23–26 °C; survives 2–36 °C. Eats bottom animals.
• Chinese carps: grass carp 23–28 °C eats water plants and cut grass; silver carp 23–28 °C eats phytoplankton; bighead carp 23–31 °C eats zooplankton.
• African catfish (Clarias gariepinus): 25–27 °C. It breathes air and survives oxygen down to about 1 mg/l.

Fish that eat different foods can share one pond: silver carp in the water, grass carp on plants, common carp on the bottom.

Lime a new pond on its dry bottom with 2,000 kg/ha of ground limestone on sandy soil to 4,000 kg/ha on heavy clay, 80–160 kg for 400 m², then fill. If total alkalinity is still under 25 mg/l a month or two later, lime again (FAO).

Manure 10–15 days before stocking at 10–20 kg of dry matter per 100 m², so the water is green when the fish arrive. Then, in a small tropical pond, once or twice a week per 100 m²: 4.5 kg poultry droppings, 3 kg sheep or goat dung, 6 kg pig dung or 5 kg cattle dung. Never exceed 1.2 kg of manure dry matter per 100 m² a day in the tropics, 0.6 kg in cool climates (FAO). Stop manuring when the noon water is under 20 °C, the Secchi disc reads under 40 cm, the pH at sunset is over 9 or the dawn oxygen is under 3–4 mg/l.

Keep animals beside the pond and their manure feeds it (Livestock): per 100 m², start low with 0.2–0.3 pigs, 3–5 ducks or 5–15 chickens.

Stock 2 tilapia of 5 cm or more per m²: 800 for a 20 × 20 m pond (FAO handbook).

Oxygen is lowest at sunrise: plankton makes it by day and uses it all night, and the richer the pond the deeper the fall. Minimum dissolved oxygen for adults: tilapia 2 mg/l, common carp 3 mg/l, African catfish 1 mg/l or less (FAO).

Make a Secchi disc: a 25 cm disc painted in black and white quarters, weighted below, on a line knotted every 10 cm. Read it between 09:00 and 15:00 on a calm day, looking straight down with the sun behind you.

• Under 25 cm: high risk of an oxygen crash, worst after 2–3 cloudy days.
• Under 40 cm: too much plankton; stop manuring.
• 40–60 cm: best production.
• Over 60 cm: too little plankton for the fish to eat; manure more.

Walk the pond at dawn. Fish gulping at the surface (piping), tadpoles crowding the edge, snails climbing stems or a smell of rotten eggs mean low oxygen. Stop feeding and manuring, let in fresh water, and run bottom water out through the monk; in a small pond, splash the surface with paddles.

Feed no more than 3 % of the fishes' weight at one meal, in the early morning and a couple of hours before sunset. With a good plankton bloom, tilapia need as little as half the feed (FAO). Skip feeding when the water is too hot or cold, when oxygen is low, and on manuring days.

Harvest at about 6 months, when most tilapia are big enough. From the fifth month, take a few big fish each week with a seine of 3–3.5 cm mesh, which lets small fish escape (FAO handbook).

Stop feeding 2–3 days before draining. Start early in the morning while it is cool, lower the water slowly so the fish follow it to the catch basin, and lift the monk's boards one row at a time, putting the screen back each time. Rinse muddy fish in clean water. Harvest only what you can eat, sell or preserve, and take the rest in later partial harvests.

Cook every fish through. Raw or undercooked freshwater fish carry the liver flukes Clonorchis sinensis and Opisthorchis viverrini; long infection causes bile-duct cancer, and WHO classes both as carcinogens (WHO). The fishing guide covers the other fish hazards.

Dry the bottom before refilling: at least 2–3 weeks in the tropics, 3–4 months in temperate climates, until the soft mud turns to soil. Dig out mud that has grown thick and spread it on crops. Each year, lime the dry bottom with a quarter of the new-pond dose.