Ammonia toxicity to freshwater fish

In response to the recent thread regarding fish mortality and water 
changes, I would like to offer some relevant information about ammonia 
toxicity and pH.  Ammonia and ammonium are in equilibrium. The balance 
shifts towards the more toxic form when the pH goes up. This can happen 
during a water change, or when CO2 is discontinued. As another poster 
mentioned, LOWER the pH when you suspect an ammonia problem.
Here is a more complete discussion which I previously published in 
Freshwater and Aquarium Fish Magazine in September 1991.

                The Effects of pH and Temperature

The topic of ammonia toxicity was thoroughly reviewed by Garrett
Glodek in the June 1991 issue of FAMA.  This article will expand
on his discussion, focusing on the effects of pH and temperature.

The term ammonia refers to two chemical species of ammonia 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.

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                          pH
                6.5      7.0     7.5      8.0     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

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)

                         total   NH3

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 published
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.


US Environmental Protection Agency.  Ambient Water Quality
Criteria for Ammonia, (EPA 440/5-85-001). January 1985.

Glodek, Garrett S. "Ammonia in the Closed System Aquarium," 
FAMA, June 1991.