pH and Ammonia

Part of Aquaculture

Water chemistry is invisible to the eye but felt by every fish in your pond. pH and ammonia are the two parameters most likely to silently kill fish in an otherwise well-managed aquaculture system. Understanding them requires no laboratory — just observation, simple tests, and corrective actions that use materials available from any farm.

A pond can look clear and healthy while its chemistry is actively poisoning fish. Mortality from poor water chemistry is often mistaken for disease. Fish become lethargic, stop eating, develop red streaks on fins, then die — symptoms that look like infection but stem entirely from water quality. Catching these problems early, and fixing them with lime, water exchange, or feeding adjustments, costs almost nothing. Ignoring them costs your whole crop.

Understanding pH in Fish Ponds

pH measures hydrogen ion concentration on a scale of 0–14. Water below 7 is acidic; above 7 is alkaline; 7 is neutral.

Optimal ranges for common aquaculture species:

Species	Optimal pH	Tolerable range
Common carp	7.0–8.5	6.0–9.0
Tilapia	6.5–8.5	5.0–10.0
Catfish	6.5–8.0	5.5–9.0
Trout	6.5–8.0	5.5–9.0

Why pH Swings Happen

The primary cause of pH swings in fish ponds is algae photosynthesis. During the day, algae consume carbon dioxide (CO₂), which raises pH. At night, algae and fish respire, releasing CO₂, which lowers pH. In a heavily planted or algae-rich pond, this daily swing can span 2–3 pH units — from 6.5 at dawn to 9.5 by afternoon.

At pH above 9, fish experience alkali burns to gills and skin. At pH below 6, gill function deteriorates, mucus production increases, and fish become prone to secondary infections.

The swing is dangerous even if the midday average looks fine. A pond that reads 8.0 at 2 PM may have been at 6.2 at 6 AM, stressing fish all night.

Testing pH Without Equipment

Litmus or pH paper: The simplest tool. Strips change color based on pH. Calibrate against a known reference (rainwater is typically 5.5–6.5; fresh lime solution is strongly alkaline).

Red cabbage indicator: Boil red cabbage in water. The resulting purple liquid turns red in acid and green/yellow in alkali. Not precise, but distinguishes strongly acid, neutral, and strongly alkaline conditions.

Observation proxy: If algae are very dense (bright green water, low visibility), afternoon pH is likely high. If you see fish piping at surface at dawn, pH may be critically low. Both scenarios warrant action.

Correcting Low pH (Acidic Water)

The most common fix is agricultural lime (calcium carbonate, CaCO₃). This is the same lime spread on fields. It neutralizes acidity and buffers against future swings.

Liming rates:

• Slightly acid (pH 6.0–6.5): 50–100 kg/hectare
• Moderately acid (pH 5.5–6.0): 100–200 kg/hectare
• Strongly acid (< 5.5): 200–500 kg/hectare, applied in stages

Spread dry lime evenly over the pond surface or dissolve it in water and pour over pond. Do not use slaked lime (calcium hydroxide, quicklime) directly — it is far more reactive and can temporarily spike pH above 12, killing fish instantly. If only slaked lime is available, dilute thoroughly in a large tank of water and apply very slowly.

Hardwood ash: High in potassium and calcium carbonates, wood ash raises pH and adds minerals. Use 50–150 kg/hectare. Less precise than agricultural lime but widely available.

Correcting High pH (Alkaline Water)

High pH from algae blooms is more difficult to correct directly. Options:

• Reduce algae: Reduce feeding (less nutrient input), add shade (floating vegetation, bamboo shade frames), or introduce filter-feeding fish (silver carp).
• Water exchange: Dilute with fresh water.
• CO₂ injection: If available (from fermentation, a compressed gas source), bubbling CO₂ into water lowers pH directly. This is rarely practical without modern equipment.
• Organic matter: Adding compost or straw temporarily acidifies as it decomposes. Use cautiously — it also consumes oxygen.

Understanding Ammonia

Ammonia (NH₃) is produced continuously by fish as they excrete nitrogen through their gills and in urine. In a healthy pond with sufficient biological filtration, bacteria convert ammonia through nitrite to relatively harmless nitrate. When this bacterial capacity is overwhelmed — by overstocking, overfeeding, sudden temperature changes, or medication that kills bacteria — ammonia accumulates.

Total Ammonia Nitrogen (TAN)

Ammonia in water exists in two forms:

• Ionized ammonia (NH₄⁺): Relatively harmless to fish at typical concentrations.
• Un-ionized ammonia (NH₃): Highly toxic. Damages gill tissue, inhibits enzyme activity, causes neurological damage.

The critical variable is that the proportion of toxic NH₃ increases sharply with rising pH and temperature.

Percentage of total ammonia that is toxic NH₃:

Temperature	pH 7.0	pH 7.5	pH 8.0	pH 8.5
20°C	0.4%	1.2%	3.8%	11.0%
25°C	0.6%	1.8%	5.5%	15.5%
30°C	0.9%	2.7%	8.0%	21.0%

Implication: At high pH and temperature — exactly the conditions of a summer algae bloom — even moderate total ammonia becomes acutely toxic. A reading of 5 mg/L total ammonia at pH 8.5 and 30°C means over 1 mg/L of toxic NH₃, which is lethal to most species.

Safe thresholds: Keep un-ionized ammonia (NH₃) below 0.02 mg/L for sensitive species (trout), below 0.05 mg/L for moderate species (carp, tilapia). Total ammonia should stay below 1 mg/L at typical summer pond conditions.

Testing Ammonia Without a Kit

Standard ammonia test kits use Nessler’s reagent or a salicylate reaction, turning yellow-orange with increasing ammonia concentration. Without a kit:

Biological indicators:

• Fish flashing (rubbing against objects) and excessive mucus production suggest gill irritation from ammonia.
• Erratic swimming, spiral motions, and loss of equilibrium at low stocking rates (where oxygen is not limiting) suggest ammonia toxicity.
• Increased susceptibility to bacterial infections (red sores, fin rot) in a well-oxygenated pond suggests chronic sub-lethal ammonia stress.

Smell: Strong ammonia odor from water or dead fish suggests very high levels, but by the time it’s detectable by smell, the crisis is severe.

Correcting High Ammonia

Immediate actions:

1. Stop feeding entirely. Every gram of protein fed produces ammonia. A complete 48-hour fast in a crisis is always correct.

2. Partial water exchange: Replace 20–30% of pond water with clean, ammonia-free water. This dilutes the problem directly.

3. Lower pH: Since pH determines how much ammonia is in toxic form, reducing pH from 8.5 to 7.5 cuts toxic fraction by 80%. Add CO₂ or use the organic matter approach carefully.

4. Increase aeration: Ammonia also off-gasses from agitated water surfaces. Vigorous aeration removes some ammonia directly.

5. Zeolite: If available (a common volcanic mineral), zeolite adsorbs ammonia from water. Crushed zeolite spread over the pond or placed in mesh bags at water inflow can temporarily buffer ammonia. Recharge by soaking in saltwater (2% NaCl) and rinsing.

Long-term management:

• Maintain feeding at rates the fish will consume within 15–20 minutes. Remove uneaten feed immediately.
• Ensure adequate biological filtration bacteria are established (they colonize pond sediment naturally over weeks to months).
• Avoid antibiotics unless absolutely necessary — they destroy nitrifying bacteria.
• Do not overstock. Calculate based on realistic carrying capacity: 1–2 kg/m² without aeration and filtration.

Integrated Monitoring Routine

Effective water quality management combines simple tests with scheduled observation:

Time	Action
Dawn (5–6 AM)	Check for fish piping (low oxygen or low pH). Observe behavior.
Mid-morning	Note water color. Green bloom indicates high afternoon pH risk.
Afternoon	Check for lethargy, erratic swimming (possible ammonia/high pH).
After storms	Check for pond turnover (milky water, sulfur smell).
Weekly	pH test, observation of feeding response, visual clarity check.

Hardness and Alkalinity

Two related parameters matter for pH stability:

Total hardness (calcium + magnesium): Ponds with hard water (> 50 mg/L as CaCO₃) are more stable. Soft water ponds (< 20 mg/L) are prone to extreme pH swings. Regular liming keeps hardness adequate.

Total alkalinity: Alkalinity (bicarbonate and carbonate concentration) acts as a buffer, resisting pH change. Ideal alkalinity: 75–150 mg/L as CaCO₃. Low alkalinity means pH swings wildly; high alkalinity resists correction.

Test alkalinity by titrating a water sample with dilute acid (vinegar) using an indicator dye, or simply observe pH stability over 24 hours. Large swings (more than 1.5 pH units from dawn to afternoon) indicate low alkalinity — lime is the fix.

Building Knowledge Over Seasons

Each pond behaves differently based on its geology, watershed, fish density, and feeding practices. Keep a simple log:

• Date, water color, observed fish behavior, feeding response
• Any corrective actions taken (lime added, water exchanged, feeding stopped)
• Weather (cloud cover, temperature, rain)

After two or three seasons you will be able to predict problems before they occur from patterns in your log — recognizing that three consecutive hot, calm days after a heavy algae bloom means you need to aerate at dawn on day four.

Water chemistry mastery does not require laboratory equipment. It requires observation, a systematic response to warning signs, and the habit of acting before problems become crises.