Fish: Water Quality, Feed Arithmetic and the Gate Sequence

#Safe at seven in the morning, lethal by four in the afternoon

The last lesson said two facts decide a fish venture: what the law permits, and what your water does in July. Suppose both came back green. Most ventures die here instead, for two reasons that have little to do with fish husbandry as people picture it. A pond's chemistry moves through the day, so water that was harmless at first light can kill by late afternoon. And at retail prices, the feed costs more than the fish sells for.

#Oxygen is the residual you cannot see

Grazing has an aquatic version: dissolved oxygen is the residual, the daily minimum you must never graze down.

The South African Water Quality Guidelines, Volume 6 set targets by species group: 6–9 mg/L for cold-water species such as trout, 5–8 mg/L for the intermediate group — carp and bass — and 5–8 mg/L for warm-water species such as tilapia and African sharptooth catfish. Between 1 and 3 mg/L is the limiting range; tilapia growth is impaired between 1 and 5 mg/L, and long exposure at 0.3–0.75 mg/L is lethal. Above about 20 mg/L the problem inverts: supersaturation causes gas-bubble disease and mortality.

Then the physics that makes summer, not winter, the dangerous season. Fresh water holds 11.29 mg/L of oxygen at 10 °C but only 7.56 mg/L at 30 °C — and fish oxygen demand doubles or trebles for every 10 °C rise. Hot water holds less oxygen and the fish want more of it. That is why a pond crashes before dawn in midsummer, not in the cold.

Sharptooth catfish tolerate low oxygen far better than tilapia — the strongest practical argument for Clarias in a low-technology system.

#Ammonia, and why the afternoon is the enemy

Fish excrete ammonia. In water it sits in two forms — harmless ammonium and toxic un-ionised ammonia (NH₃) — and pH and temperature set the split. The targets are far apart: 0.0–0.025 mg NH₃/L for cold-water fish and 0.0–0.3 mg NH₃/L for warm-water fish.

Here is the mechanism behind dead fish in the afternoon after a clean morning check. Through the day, algae strip carbon dioxide out of the water. pH climbs. Water warms. The same total ammonia that sat almost entirely as harmless ammonium at 07h00 has shifted towards un-ionised NH₃ by 16h00. Nothing was added; the pond moved. The guideline's mitigations — fresh water, biological filtration, lower pH, lower temperature, more oxygen — are mostly things you design before stocking.

#The arithmetic that closes the argument

Work it on a thousand kilograms, all at July 2026 prices. At FCR 1.5 you buy 1,500 kg of feed at R38/kg: R57,000. Sell all 1,000 kg direct at R70/kg and you gross R70,000. Margin over feed alone: R13,000 — before fingerlings, power, labour, the pond, security and mortality. One 30% die-off puts you under for the year. Now change one variable: sell into a wholesale or frozen channel competing with imports that landed at R12.39/kg in 2023 and revenue becomes R12,390 against the same feed bill.

So the only fish that pays in South Africa is live or fresh fish, sold locally within a day, at a real retail price. The national picture fits that arithmetic. Of the aquaculture farmers DFFE surveyed for 2023, 52% reported annual income under R100,000 and not one reported over R1 million (Yearbook 2024). Say it the way DFFE says it: that is what the people who answered a voluntary questionnaire reported, not a proven ceiling. It is still the only national income picture there is, and it points the same way the feed bill does.

#Why aquaponics does not rescue the fish

The escape route offered to beginners is aquaponics: let the fish feed the vegetables and the vegetables carry the business. The best South African evidence says the second half is right and the first half is a trap.

#A permit stack with no Act at the top

There is no overarching aquaculture Act in South Africa. DFFE's own words: the freshwater sector "is regulated through provincial Acts and Ordinances, but there is no overarching legislation to govern the sector". You assemble compliance from six or seven instruments.

#A farm dam is not a closed system

The most attractive proposition here — stock a dam you already own — carries one risk that is easy to talk past.

Epizootic ulcerative syndrome (EUS) is a water mould with little host specificity, reported from South Africa in 2011 in two events (Huchzermeyer & van der Waal 2012). At Arrieskraal Dam in February 2011, an impoundment on the Palmiet River, wild fish went first; farmed rainbow trout in the same water appeared unaffected, then developed lesions. In October 2011, in a farm dam on the Eerste River, EUS "caused serious mortality amongst free-living sharptooth catfish".

Three things follow, and the third stings. Both outbreaks were in impoundments connected to a river — stocking a dam does not isolate your fish from the catchment, or the catchment from yours. Farming in water that holds wild fish means sharing their diseases. And the documented South African mass mortality killed sharptooth catfish, the species this course recommends as the least-bad warm-water choice for a Highveld dam. Least bad is not safe.

Whether EUS has been found here since 2011 could not be established for this course, and the duty does not depend on knowing: buy fingerlings only from accredited hatcheries, quarantine incoming batches, never move live fish between water bodies, and report mass mortality or ulcerated fish to your state vet.

#Systems, and what each really costs

• Earth ponds — lowest capital, control and yield. Workable for catfish and carp; marginal for tilapia outside the warm north and east.
• An existing farm or irrigation dam — genuinely regenerative, because the capital is already sunk. The only configuration most readers should consider.
• Cages in a dam — moderate capital, concentrated nutrient load, needs through-flow. Theft and predation are the killers, and above 20,000 kg/yr it is an EIA.
• Raceways — need cold clean water by gravity, so in practice the cold high-flow catchments, and trout.
• Recirculating systems (RAS) — highest capital and control, and fundamentally an electricity-dependency machine. Hours without power kills the biomass, so size it around backup power, not fish.

#Five gates, and the honest no

Run them in order and stop at the first no. Most people should stop.

Run the gates this month, on paper, before you move a cubic metre of soil. Past Gate 5, build small in a dam you already own and sell live at the farm gate. Short of it, note which gate stopped you and spend the money on winter forage instead.