Ammonia is toxic to fish if allowed to accumulate in fish production systems. When ammonia accumulates to toxic levels, fish cannot extract energy from feed efficiently. If the ammonia concentration gets high enough, the fish will become lethargic and eventually fall into a coma and die.
In properly managed fish ponds, ammonia seldom accumulates to lethal concentrations. However, ammonia can have so-called “sublethal” effects—such as reduced growth, poor feed conversion, and reduced disease resistance—at concentrations that are lower than lethal concentrations.
Source of ammonia
The main source of ammonia in fish ponds is fish excretion. The rate at which fish excrete ammonia is directly related to the feeding rate and the protein level in feed. As dietary protein is broken down in the body, some of the nitrogen is used to form protein (including muscle), some is used for energy, and some is excreted through the gills as ammonia. Thus, protein in feed is the ultimate source of most ammonia in ponds where fish are fed.
Another main source of ammonia in fish ponds is diffusion from the sediment. Large quantities of organic matter are produced by algae or added to ponds as feed. Fecal solids excreted by fish and dead algae settle to the pond bottom, where they decompose. The decomposition of this organic matter produces ammonia, which diffuses from the sediment into the water column.
Ammonia management options
Stop Feeding or Reduce Feeding Rate
The primary source of nearly all the ammonia in fish ponds is the protein in feed. When feed protein is completely broken down (metabolized), ammonia is produced within the fish and excreted through the gills into pond water. Therefore, it seems reasonable to conclude that ammonia levels in ponds can be controlled by manipulating feeding rate or feed protein level. This is true to some extent, but it depends on whether you want to control it over the short-run (days) or the long-run (weeks or months).
In the short-run, sharp reductions in feeding rate have little immediate effect on ammonia concentration. The ecological reason for this is based on the complex movement of large amounts of nitrogen from one of the many components of the pond ecosystem to another. In essence, trying to reduce ammonia levels by withholding feed can be compared with trying to stop a fully loaded freight train running at top speed—it can be done but it takes a long time.
Producers can reduce the risk over the long-run by adjusting both feeding rate and feed protein level. Limit feed to the amount that will be consumed. In mid-summer the maximum daily feeding rate should be 100 to 125 pounds per acre. By feeding conservatively, the potential for high ammonia in ponds and the risks associated with sub-lethal exposure (disease, poor feed conversion, slow growth) can be minimized.
The toxic form of ammonia (NH3) is a dissolved gas, so some producers believe pond aeration is one way to get rid of ammonia because it accelerates the diffusion of ammonia gas from pond water to the air. However, research has demonstrated that aeration is ineffective at reducing ammonia concentration because the volume of water affected by aerators is quite small in comparison with the total pond volume and because the concentration of ammonia gas in water is typically fairly low (especially in the morning). Intensive aeration may actually increase ammonia concentration because it suspends pond sediments.
It has long been thought that liming ponds decreases ammonia concentrations. In fact, using liming agents such as hydrated lime or quick lime could actually make a potentially bad situation much worse by causing an abrupt and large increase in pH. Increasing pH shifts ammonia toward the form that is toxic to fish. In addition, the calcium in lime can react with soluble phosphorus, removing it from water and making it unavailable to algae.
In ponds with similar algal density, daily fluctuations of pH in lowalkalinity pond waters are more extreme than those in waters of sufficient alkalinity (greater than 20 mg/L as CaCO3; see SRAC Publication No. 464). Therefore, liming can moderate extreme pH values, particularly those that occur during late afternoon when the fraction of total ammonia that is in the toxic form is highest. However, this technique is effective only in ponds with low alkalinity. Most fish ponds have sufficient alkalinity. Increasing the alkalinity above 20 mg/L as CaCO3 will not provide additional benefit. Furthermore, liming does not address the root causes of high ammonia concentration; it only shifts the distribution of ammonia from the toxic to the non-toxic form by moderating high pH in the afternoon.
Fertilize with Phosphorus
Most of the ammonia excreted by fish is taken up by algae, so anything that increases algal growth will increase ammonia uptake. This fact is the basis for the idea of fertilizing ponds with phosphorus fertilizer to reduce ammonia levels. However, under “normal” pond conditions, algae blooms in fish ponds are very dense and the rate of algae growth is limited by the availability of light, not nutrients such as phosphorus or nitrogen. Therefore, adding phosphorus does nothing to reduce ammonia concentration because algae are already growing as fast as possible under the prevailing conditions.
The highest ammonia concentrations in fish ponds occur after the crash of an algae bloom. Fertilization, particularly with phosphorus, may accelerate the re-establishment of the bloom, but most ponds have plenty of dissolved phosphorus (and other nutrients) to support a bloom and do not need more.
Reduce Pond Depth
Algal growth (and therefore the rate of ammonia uptake by algae) in fish ponds is limited by the availability of light. Anything that increases light increases ammonia uptake. Theoretically, dense algae blooms in shallow ponds will remove ammonia more effectively than the same dense blooms in deeper ponds. On balance, however, there are probably more benefits associated with deeper ponds (e.g., ease of fish harvest, water conservation, more stable temperatures, reduced effect of sedimentation on interval between renovations).
Increase Pond Depth
Obviously, deeper ponds contain more water than more shallow ponds. Therefore, at a given feeding rate, deeper ponds should have lower ammonia concentrations because there is more water to dilute the ammonia excreted by fish. In reality, deeper ponds do not usually have enough water to significantly dilute ammonia when compared to the large amounts of ammonia in constant flux between various biotic and abiotic compartments in ponds. Furthermore, deeper ponds are more likely to stratify and the lower layer of pond water (the hypolimnion) can become enriched with ammonia and depleted of dissolved oxygen. When this layer of water mixes with surface water in a “turnover,” severe water quality problems may result.
Flush the Pond with Well Water
Ammonia can be flushed from ponds, although pumping the huge volume of water required to do so in large commercial ponds is costly, time-consuming and unnecessarily wasteful. It is also deceptively ineffective as an ammonia management tool. For example, assume the ammonia concentration in a full, 10-acre pond is 1 mg/L. The ammonia concentration after pumping 500 gpm continuously for 3 days (equivalent to about 8 inches of water) will be 0.90 mg/L, a drop of only 0.10 mg/L.
Instead of simply running water through a pond as in the example above, now assume that about 8 inches of water is discharged from the pond before refilling with well water. In this case, the decline in ammonia concentration will be slightly greater (to 0.83 mg/L), but even this decrease is not enough in an emergency situation, particularly when the extra time needed to drain the water before refilling is considered. The difference in the two flushing scenarios is related to the blending of pond water with pumped water before discharge in the first case.
Just as paddlewheel aeration creates a zone of sufficient dissolved oxygen concentration, pumping groundwater creates a zone of relatively low ammonia concentration adjacent to the water inflow. The effectiveness of this practice is questionable because it does not address the root cause of the problem and wastes water. Flushing ponds is not only ineffective, but highly undesirable because of concerns about releasing pond effluents into the environment.
Add Bacterial Amendments
Common aquatic bacteria are an essential part of the constant cycling of ammonia in a pond ecosystem. Some people believe that ammonia accumulates in ponds because the wrong kind or insufficient numbers of bacteria are present. If this were true, adding concentrated formulations of bacteria would address the problem. However, research with many brands of bacterial amendments has consistently given the same result: Water quality is unaffected by the addition of these supplements.
Standard pond management creates very favorable conditions for bacterial growth. Bacterial growth and activity is limited more by the availability of oxygen and by temperature than by the number of bacterial cells. Also, the most abundant type of bacteria in many amendments (and in pond water and sediment) is responsible for the decomposition of organic matter. Therefore, if bacterial amendments accelerate the decomposition of organic matter, ammonia concentration would actually increase, not decrease.
Another kind of bacteria in amendments oxidizes ammonia to nitrate. Adding them will not reduce the ammonia concentration rapidly because the bacteria must grow for several weeks before there is a large enough population to affect ammonia level.
Add a Source of Organic Carbon
If the dissolved oxygen concentration is adequate, adding a source of organic carbon, such as chopped hay, to intensive fish ponds can reduce ammonia concentration. Many bacteria in fish ponds are “starved” for organic carbon, despite the addition of large amounts of feed. Organic matter in fish ponds (dead algae cells, fish fecal solids, uneaten feed) does not contain the optimum ratio of nutrients for bacterial growth. There is more than enough nitrogen for bacterial growth so the excess is released to the pond water.
Adding organic matter with a high concentration of carbon relative to nitrogen promotes the “fixation” or “immobilization” of the ammonia dissolved in water. Incorporating ammonia into bacterial cells packages the nitrogen into a particulate form that is not toxic to fish. The down side of this approach is that it is hard to apply large amounts of organic matter to large ponds and the effect on ammonia concentration is not rapid. Furthermore, aeration will have to be increased to address the demand for oxygen by large quantities of decomposing organic matter.
Add Ion Exchange Materials
Certain naturally occurring materials, called zeolites, can adsorb ammonia from water. These are practical to use in aquaria or other small-scale, intensive fish-holding systems, but impractical for largevolume fish ponds.
Some shrimp farmers in Southeast Asia have tried making monthly applications of zeolite at 200 to 400 pounds per acre. However, research has demonstrated that this practice is ineffective at reducing ammonia concentration in ponds and it has now been abandoned.
In theory, adding acid (such as hydrochloric acid) to water will reduce pH. This can shift the ammonia equilibrium to favor the non-toxic form. However, a large amount of acid is necessary to reduce the pH in well-buffered ponds and it would have to be mixed rapidly throughout the pond to prevent “hot spots” that could kill fish. Furthermore, adding acid would destroy much of the buffering capacity (alkalinity) of the pond before any change in pH could occur. Once the ammonia concentration is lowered, treated ponds might require liming to restore the buffering capacity. Working with strong mineral acids is a safety hazard for farm workers and for fish.