Pond Water Quality: What Every Fish Farmer Must Know
Just before sunrise in a small pond outside Chiredzi, the water looks calm. But along the bank, tilapia are clustered at the surface, quietly gasping. By mid-morning, half the pond is floating belly-up. To the farmer, it feels sudden. In reality, the warning signs were there in the water all along.
Water quality is the single biggest factor deciding whether your tilapia grow fast and stay healthy — or die. Most problems are invisible until fish start to struggle. Learning to read a few key signs can save your crop and your cash.
The Four Key Parameters
1. Dissolved Oxygen (DO)
Tilapia need at least 3 mg/L of dissolved oxygen to survive, and 5 mg/L or more to grow well. Oxygen levels are highest in the late afternoon and drop overnight as algae and bacteria use it up. The danger time is early morning, just before sunrise.
Signs of low oxygen:
- Fish gasping at the surface in the morning.
- Fish crowding near the inlet pipe or any spot where fresh water enters.
- Reduced feeding; fish ignore feed or eat slowly.
What to do:
- Add aeration by splashing water from a height, using a pump, or using a paddlewheel aerator if you have one.
- Reduce stocking density over time if the pond is chronically low in oxygen.
- Stop feeding until oxygen recovers — uneaten feed rots and makes the problem worse.
FFA Explainer:
Oxygen enters pond water through contact with the air and from photosynthesis by algae and plants. At night, however, plants and animals consume oxygen. That is why tilapia often die in the early hours after a hot day with heavy feeding and strong algae growth.
2. pH (Acidity/Alkalinity)
pH measures how acidic or alkaline your water is. Tilapia thrive in a pH between 6.5 and 9.0.
- Below 6.0: fish become stressed, stop feeding, and are more vulnerable to disease.
- Above 10.0: ammonia in the water becomes highly toxic, even at low levels.
How to check pH:
- Use a simple pH test kit or pH strips from an agricultural supplier. Many cost less than US$5.
- Test in the afternoon, when pH is at its daily peak.
What to do:
- If pH is too low, apply agricultural lime (calcium carbonate) at 100–200 kg per hectare of pond surface. Spread evenly and apply gradually over several days.
- If pH is too high, reduce fertiliser applications and avoid over-fertilising the pond.
3. Ammonia
Ammonia is released from fish waste and decomposing feed. You can’t see it, but at high levels it burns fish gills and can wipe out a pond overnight.
High ammonia usually comes from:
- Overfeeding — too much feed sinking and rotting.
- Overcrowding — too many fish producing waste.
- Poor or no water exchange.
Signs of ammonia problems:
- Fish appear sluggish and spend more time near the surface.
- Fish stop eating, or feeding activity drops sharply.
- Reddening or irritation around the gills.
Test strips can confirm if ammonia is high, but behaviour is often your first warning.
What to do:
- Increase water exchange immediately if you have a reliable water source.
- Reduce feeding by at least 50% for three to four days.
- Remove all dead fish as soon as you see them; rotting carcasses spike ammonia quickly.
4. Turbidity (Cloudiness)
A healthy tilapia pond usually has a light green colour. This shows a good bloom of phytoplankton — tiny plants that produce oxygen and serve as natural food.
Muddy brown or grey water means clay or soil particles are suspended in the water. This reduces light penetration, cuts phytoplankton growth, and can reduce feeding.
The Secchi disc test:
- Fix a white disc (or a painted white metal/plastic plate) to a string.
- Lower it into the pond until you just can’t see it.
- Measure the depth.
Interpretation:
- If the disc disappears at 20–40 cm depth, turbidity is in a healthy range.
- Less than 20 cm: water is too “thick” — risk of low oxygen at night.
- More than 40 cm: pond may be under-fertilised; natural food and oxygen production are low.
FFA Explainer:
Turbidity can come from algae (green) or from soil (brown/grey). Algae-based turbidity is usually good within limits. Soil-based turbidity blocks sunlight without feeding fish or producing oxygen. Controlling erosion around ponds and avoiding animals entering the water help keep soil out.
Daily Management Habits
Good water quality is built on small, daily routines rather than expensive equipment.
- Check fish at first light every morning. Behaviour is your best early warning system. Look for gasping, unusual swimming, or fish crowding in one area.
- Remove dead fish immediately. Record the date and number in a notebook. Rising deaths often signal water or disease problems.
- Never feed stressed fish. If fish are gasping, sluggish, or behaving abnormally, fix the water first. Extra feed will only rot and poison the pond.
- Keep simple records. Note weather, water colour, fish behaviour, and any unusual events. Patterns over days and weeks help you predict problems before they explode.
Farmers from districts such as Kariba, Goromonzi, and Gweru who adopt a “five-minute morning check” often avoid the sudden mass deaths that hit unmonitored ponds.
Water quality may be invisible, but its effects are not. Treat your pond like a living system, pay attention to these four key parameters, and your tilapia are far more likely to reward you with strong, healthy growth instead of costly surprises.
Article prepared by FFA Research Team | Farm Future Africa | farmfutureafrica.co.zw
📲 Subscribe to our WhatsApp Channel to Get Our Updates On Your Mobile
About The Author
