It's important to know about this danger in order to maintain well-functioning aquarium with healthy fish.
When ammonia and/or nitrite levels in an aquarium become too high, such as during the period when biological filtration is being established, fish become sick and frequently die. Ammonia toxicity and nitrite toxicity are the result of complex interactions between these chemicals, other water quality parameters and the fish.
Ammonia toxicity and nitrite toxicity are two of the leading causes of fish death in aquarium systems. At higher levels, effects are immediate and numerous deaths can occur rapidly. However, lower levels, over a long period of time, can still result in increased incidence of disease and more chronic losses. Other water quality parameters - including pH (a measure of the amount of acid in water), temperature and dissolved oxygen - determine the degree of toxicity.
Ammonia in water exists in two forms: unionized ammonia (UIA); and ionized ammonia (IA). UIA is considered much more toxic than IA. Most test kits measure total ammonia nitrogen (TAN). TAN is the sum of the two forms (UIA and IA).
So what determines whether there will be more UIA or more IA in any given system? The two major factors are temperature and pH. As temperature or pH increase, so does the proportion of UIA. The following examples should help explain these concepts:
Let's say we have two tanks, Tank A and Tank B. Both tanks have a pH of 7.8 and a TAN of 1.0 part per million (ppm or milligrams per liter of water). Tank A has a temperature of 22 degrees Celsius (about 72 degrees Fahrenheit) and Tank B has a temperature of 26 C (about 79 F). Which tank has more of the toxic ammonia form (UIA)?
Tank B does because it has a higher temperature (remember, both have a pH of 7.8). About 2.8 percent of Tank A's TAN is toxic ammonia, while Tank B has about 3.7 percent.
Let's say we have two other tanks, Tank C and Tank D. This time, both tanks have the same temperature, 26 C (about 79 F) and the same TAN, about 1.0 ppm. However, Tank C has a pH of about 7.0, while Tank D has a pH of about 8.0. Which tank has more toxic ammonia (UIA)?
Tank D does, because it has a higher pH (remember, both have a temperature of 26 degrees C). Tank C has about 0.6 percent toxic ammonia, while Tank D has about 5.7 percent toxic ammonia. Tank D has almost 10 times more toxic ammonia than Tank C!
Toxic Effects of Ammonia
Excess ammonia causes many problems in fish. One significant effect is damage to the gills. Although the most obvious consequence of this is impaired respiration (breathing) this isn't the only problem. Gills are also important for acid-base balance (keeping the pH of the fish's blood correct to allow for normal processes to occur) and ion exchange (keeping the correct amount of important ions such as sodium and chloride in the blood). Thus, damage to the gills prevents a number of important processes from occurring. This leads to extra stresses on the fish as well as an increased potential for infection by bacteria and other invaders.
Ammonia also causes damage to skin, fins and the intestine. More chronic ammonia exposure can cause kidney damage, decreased growth and overall immune suppression. Ammonia also affects the nervous system, resulting in erratic swimming behavior.
What to Watch For
Nitrite binds to red blood cells and blocks their ability to transport oxygen. If nitrite is present in high enough concentration, fish blood will turn a chocolate-brown color. Because of this, nitrite toxicity is also known as brown blood disease. This effect is most easily seen in the gills. However, toxicity can and does occur commonly at lower concentrations, before gills have turned an obvious brown.
Because a fish's red blood cells with nitrite present cannot transport oxygen, affected fish frequently appear to be oxygen-deprived, even in water with high concentrations of oxygen. Affected fish may gasp at the surface or stay near airlines or water inflow. If nitrite is present at lower levels, fish may only show signs of toxicity when they are stressed further and require more oxygen. Over longer periods of exposure to nitrite, fish can become anemic (i.e., deficient in red blood cells).
Prevention of Ammonia and Nitrite Toxicity
The best way to avoid ammonia and nitrite toxicity is by making sure the biological filter is well established and maintained. This means having patience when first starting up an aquarium and testing water quality, especially ammonia and nitrite levels regularly.
When increases or spikes of ammonia occur, water changes may be the only means to lower ammonia concentrations, until the cause of the spike has been corrected.
Water changes may be the only temporary solution for nitrite spikes as well. However, another temporary solution is the addition of salt (sodium chloride) in proper proportion to the tank. A chloride to nitrite ratio of 6:1 has been recommended to decrease toxic effects. The addition of chloride during a nitrite spike helps prevent entry of nitrite into the fish's blood stream.