The ammonium ion.
Ammonia (NH3) has
gained an extra hydrogen together with a positive charge
Ammonia - the silent killer
Ammonia is extremely toxic and even relatively low levels pose a
threat to fish health. Ammonia is produced by fish and all other
animals, including ourselves, as part of normal metabolism. Such is the
toxicity, that most animals immediately convert it to a less harmful
substance, usually urea, and excrete it in urine.
Fish shortcut this process and continually excrete metabolic ammonia
directly into the surrounding water via special cells in the gills. In a
natural environment, such as seas, lakes and rivers, it would be
immediately diluted to harmless levels. However, in the confines of
aquaria and ponds, levels can rapidly rise to dangerous levels unless it
is constantly removed, usually by biological filtration. Additional
amounts are produced from decomposing fish food, fish waste and
The effects on fish health
Raised levels affect fish health in several different ways. At low
levels (<0.1 mg/litre NH3)
it acts a strong irritant, especially to the gills. Prolonged exposure
to sub-lethal levels can lead to skin and gill hyperplasia . Gill
hyperplasia is a condition in which the secondary gill lamellae swell and
thicken, restricting the water flow over the gill filaments. This can
result in respiratory problems and stress and as well as creating
conditions for opportunistic bacteria and parasites to proliferate.
Elevated levels are a common precursor to bacterial gill disease.
Fish response to sub lethal levels are similar to those to any other
form of irritation, i.e. flashing and rubbing against solid
objects. Without water testing it would be very easy to wrongly conclude
the fish had a parasite problem.
click on pictures to enlarge them
Fish gill with
hyperplasia. The gill filaments are swollen and clumped
together, reducing the fish's ability to 'breath'.
At higher levels (>0.1 mg/litre NH3) even relatively short exposures can
lead to skin, eye, and gills damage. Elevated levels can also lead to
ammonia poisoning by suppressing normal ammonia excrement from the
gills. If fish are unable to excrete this metabolic waste product there
is a rise in blood-ammonia levels resulting in damage to internal
The fish response to toxic levels would be lethargy, loss of
appetite, laying on the pond bottom with clamped fins, or gasping at the
water surface if the gills have been affected. Because this response is
similar to the response to poor water quality, parasite infestations and
other diseases, it is important that a proper investigation is made to
establish the real cause before administering
any treatments that may exacerbate the problem.
The chemistry of ammonia
When dissolved in water, normal ammonia (NH3) reacts to
form an ionised species called ammonium (NH4+)
NH4+ + OH-
This is a shorthand way of saying that one molecule of ammonia reacts
with one molecule of water to form one ammonium ion and a hydroxyl ion.
From the doubled headed arrow we can tell that the reaction can go
either way and hydroxyl ions and ammonium ions could combine to form
ammonia and water. This is precisely what happens as the pH of water
increases; that is the water becomes more alkaline. You may recall that
alkalinity is caused by an increase in hydroxyl ions. An increase in
hydroxyl ions (or alkalinity) pushes the equilibrium to the left and
more unionised ammonia is formed.
At any given time there will be both ammonia molecules and ammonium
ions present. The quantity of each species is dependant on both pH and
The toxicity of ammonia
As we have already said, ammonia (NH3) is highly toxic,
whereas the ammonium ion is significantly less toxic. All test kits
measure total ammonia-nitrogen (TAN), that is ammonia plus ammonium.
However it is possible to determine the actual ammonia level if we know
(a) TAN, (b) the water temperature and (c) the water pH.
Using this information we can then calculate the percentage of
un-ionised ammonia at any given pH and temperature. Table 1 below shows
how changes in pH and temperature affect the toxicity of TAN.
Table 1 Showing the maximum levels of total
ammonia (TAN mg/litre) for fish health
Table 1 shows the maximum acceptable level of TAN at a given pH and
temperature. For example at pH 7.5 and a water temperature of 20oC
a TAN of 2 mg / litre would be fairly safe as only 1.2% would exist
as un-ionised ammonia (0.024 mg/litre). However, the danger is
that should the pH or temperature rise, this ‘safe’ level would
quickly become toxic!
As a rule of thumb it is best to aim for a zero level of total
ammonia at all times. In normal circumstances any readings above 0.1
mg/litre TAN should be considered as unacceptable and steps taken to
New pond or tank syndrome
Elevated levels are common when people set up new ponds or tanks.
Biological nitrification is a bit of a chicken and egg situation,
inasmuch that nitrifying bacteria will not multiply and grow until the
ammonia or nitrite level in the water rises. Because nitrifying bacteria
are slow growers it can take several weeks, even at reasonable
temperatures, for the numbers to increase to a point where they can ‘process’
the ammonia through to nitrate. Until the nitrifiers are well
established, ammonia, and later nitrite, levels may be unacceptable and
a threat to fish health.
The first stage in nitrification is ammonia being converted to less
toxic nitrite (NO2-) by Nitrosomonas sp. As the
population of Nitrosomonas grows, the ammonia level starts to
drop. However, this is usually followed by an increase in nitrite
levels, which persists until the population of a different bacterium, Nitrobacter
sp. reaches optimum levels. Nitrobacter convert nitrite to
nitrate which is usually considered harmless at levels less than 50
Ammonia levels in a new set-up can be minimised by only introducing a
few new fish at a time and allowing the nitrifiers to ‘catch up’
with the increase in ammonia before putting more fish in. There are
bacterial cultures available that are supposed to speed up filter
maturation, but even with these there will still be a time lag before
nitrification cuts in.
Problems with established set-ups.
Once a well-planted garden pond is established there are unlikely to
be any ammonia problems – unless it is overstocked! However, there is
always a risk of elevated levels with aquaria and koi ponds that are
heavily dependant on biological filtration to maintain water quality.
This can happen if;
The filtration is inadequate for the
If the system is poorly maintained
with large amounts of decomposing matter producing additional
ammonia or inhibiting the nitrifying bacteria
If the nitrifying bacteria have been
affected by any chemical treatments or pollutants
Depending on the cause, it is vital to rectify the underlying problem
by either improving the filtration or regular maintenance. In the
meanwhile until zero-ammonia conditions are restored the water needs to
be managed as suggested below
Reducing ammonia levels
If levels do start to increase they can be reduced by;
Partial water changes on a daily basis until an acceptable
level is obtained
Reduce or stop feeding
Using zeolite or some other form of
ion exchanging material. These act as a magnet and swap ammonium
molecules in the water for another ion, usually sodium. They need
to be re-charged, usually by overnight immersion in a strong salt
solution. Zeolites cannot be used with salted water.
It should be remembered that the aim of this management is to
reduce the ammonia to an acceptable level – not zero levels, as
a continuous supply of ammonia is needed to encourage the growth
of nitrifying bacteria!
Where there is an existing problem and especially with new set-ups it
is vital to test for ammonia on a frequent basis. I would suggest daily
until things started to stabilise.
In an established system weekly testing should be the norm. At the same time as testing for
ammonia, it is important to start testing for nitrite (NO2-) as this is the next
stage of nitrification, with ammonia being converted to nitrite. As the
ammonia levels start to stabilise we would expect to see an increase in
nitrite levels – which in turn has to be managed until the Nitrobacter
bacteria that process nitrite become fully established.