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Ammonia Toxicity to Freshwater Fish
the effects of pH and temperature

Neil Frank <Neil.Frank-at-lambada.oit.unc.edu>

The term ammonia refers to two chemical species which are in equilibrium in water (NH3, un-ionized and NH4+, ionized). Tests for ammonia usually measure total ammonia (NH3 plus NH4+). The toxicity to ammonia is primarily attributable to the un-ionized form (NH3), as opposed to the ionized form (NH4+). In general, more NH3 and greater toxicity exists at higher pH. However, limited data also indicate that less NH3 is needed at lower pH to produce its toxic effects. For the remainder of this discussion, NH3 always refers to un-ionized ammonia.

percent ammonia (as NH3) increases with pH and temperature

For a given pH and temperature, the percent of NH3 can be determined (EPA, 1985). Percent NH3 increases with temperature and pH. Some relevant numbers for most freshwater aquarium fish are presented in Table 1.

Table 1. Un-ionized NH3 as a percent of total ammonia (by temperature and pH).
Percent NH3 of total ammonia
Temp (F) pH 6.5 pH 7.0 pH 7.5 pH 8.0 pH 8.5
68 .13 .40 1.24 8.82 11.2
77 .18 .57 1.77 5.38 15.3
82 .22 .70 2.17 6.56 18.2
86 .26 .80 2.48 7.46 20.3

percent of NH3 is very dependent on pH

You will note that NH3 is much more dependent on pH than temperature. Within the pH range shown, an increase of one pH unit will increase the NH3 concentration about 10-fold.

The USEPA publishes water quality criteria for aquatic organisms. They base these criteria on published studies on fish and other aquatic life and focus on lethal concentrations, typically the concentration at which 50 percent of the test animals die. Other studies have examined the effects at lower "sublethal" concentrations. Although most of the studies on fish deal with food fish (trout, salmon, etc.), some were based on aquarium fish such as oscars and guppies. Among the food fish, salmonids are the more sensitive, so there are separate published criteria for these fish.

EPA's criteria are presented in terms of pH and temperature for both total ammonia and un-ionized ammonia (NH3), for 1-hr values and 4-day averages. They do not publish one single number. The total and un-ionized concentrations correspond to the equilibrium percentages shown above. EPA recommends that these levels not be exceeded more than once in three years to permit a system to recover from the stress caused by the ammonia pollution. EPA recognizes that some mortality is acceptable in order to protect most ecosystems and that the criteria are inappropriate when there are sensitive, locally important organisms. For most aquarists, therefore, an additional "margin of safety" is recommended in order to avoid any mortality. For our purposes, therefore, I relabel EPA's concentrations as "Lethal Concentrations."

I would suggest concentrations no more than one tenth EPA's recommended values to establish safe levels to avoid killing our fish.

As pH and temperature decrease, more total ammonia can be tolerated. Interestingly, however, less un-ionized NH3 is needed at lower pH to be lethal.

In Table 2, I present levels for 86 degrees F. based on published values for the more sensitive salmon (EPA, 1985). These will be used as a guide of lethal concentration for aquarium fish. Because of the relationship between temperature and percent NH3, even more total ammonia can be tolerated at lower temperatures (twice as much at 68 degrees). Remember, an additional margin of safety may be needed to avoid any mortality.

Table 2. Lethal ammonia concentrations at 86 degrees F. (by pH, and duration of exposure)
pH Duration Lethal* Ammonia Concentration (mg/l)
totalNH3
6.5 1-hr 14.3 0.036
4-day 0.73 0.002
7.0 1-hr 11.6 0.093
4-day 0.74 0.006
7.5 1-hr 7.3 0.181
4-day 0.74 0.019
8.0 1-hr 3.5 0.26
4-day 0.47 0.035
8.5 1-hr 1.3 0.26
4-day 0.17 0.035
*Lethal concetrations are derived from levels at which half of the exposed individuals die.

How should we interpret these numbers?

First, the important numbers to us are total ammonia, since this is measured in our test kits. You will note that less total ammonia can safely be present at higher pH. This is because the percent of toxic un-ionized NH3 increases with pH. However, you will also notice that less concentration of this NH3 can be tolerated at lower pH.

Second, what can you expect to happen at these concentrations: fish will die! From published studies, these are concentrations at which you can expect mortality from half of the animals exposed. (In the scientific literature, these are called LC50 values.) These concentrations may cause loss of equilibrium, hyperexcitability, increased breathing, decrease in nitrogen excretion, not to mention death. At lower concentrations, ammonia also has other effects which include reduced hatch and reduced growth rates.

Since these numbers are both temperature and pH dependent, let's look at the worst case of 86 degrees and pH 8.5. Hopefully, this should satisfy almost all freshwater aquarium conditions. Furthermore, let's reduce published concentrations by an additional factor of 10 to provide a margin of safety. Then, it follows that short-term concentrations of total ammonia should not exceed 0.1 mg/l and longer term (4-day average) concentrations should be less than 0.02 mg/l.

References:

  1. US Environmental Protection Agency. Ambient Water Quality Criteria for Ammonia, (EPA 440/5-85-001). January 1985.
  2. Glodek, Garrett S. "Ammonia in the Closed System Aquarium," FAMA, June 1991.
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