Front Page ACVC Site Go to First Presentation Go to Previous Presentation Go to Next Presentation Go to Last Presentation
Back to Previous Page Print This Page Save This Page Bookmark This Page Go to the Top of the Page

Water Quality and its Impact on Diseases of Fish

Greg Lewbart M.S., V.M.D., Dipl. ACZM


In these notes, major water quality and water chemistry components will be defined and discussed. Methods of altering and controlling the various parameters will also be included. Without question the major cause of tropical fish disease and mortality is poor water quality. Some may argue the vital importance of good nutrition and how this may be more critical than quality water. While a proper balanced diet is essential, especially on a long term basis, if the water is not clean and without toxins, nutrition will not have enough "time" to be a factor. Well conditioned tropical fish in good, clean, oxygenated and temperature regulated water are generally resistant to disease. Man made aquarium systems can only approximate conditions found in nature. We use filters, pumps, heaters, lights and chemicals to mimic natural environmental conditions. When all of these aquarium components are working properly, the basic life supporting parameters can be maintained. Most of the time and money spent on aquariums and aquarium maintenance center around the concept of good clean water.

When treating or caring for pet fish we as veterinarians need to "think aquatic." Human beings can leave a smoke filled room and our bodily wastes disappear with the press of a handle. A pet fish may spend nearly its entire life in 10 gallons of water. The quality of that water dictates to a large degree how long the fish can live in that tank.

Water Chemistry

Water chemistry is the most complicated part of aquarium management and the most important. It is necessary to have an understanding of water chemistry principles in order to successfully diagnose and correct aquarium problems. There are a number of good references on the subject1,2,3,4,5,6,7

Oxygen is the most important life supporting component found in water. Water contains much less oxygen than air (0.004% in sea water and 21.0% in atmospheric air) and most fish must ventilate a volume of water 10 times that of the air ventilated by a terrestrial animal to obtain the same quantity of oxygen. The amount of oxygen dissolved in a given volume of water depends on four factors: temperature, atmospheric pressure, salinity and the aquatic plants in the system. In water, oxygen is either saturated, supersaturated or undersaturated. With adequate aeration from a good air pump most home aquariums are saturated and contain enough oxygen for the tank residents. Overaerating a tank can result in a condition known commonly as "gas bubble disease." In this situation, the tank water is supersaturated with atmospheric air and gasses come out of solution in the bloodstream and epithelial tissue of the affected fish. These emboli can kill the fish and the situation must be corrected quickly. The author has observed this problem when the tank temperature is very high (over 900 F) and the aquarist tries to compensate for the low oxygen levels by excessively aerating the aquarium. Gas bubble disease is similar to the condition know as the "bends" when nitrogen in the bloodstream of a SCUBA diver comes out of solution. Making temperature control and aeration adjustments will solve this problem. Large subcutaneous emboli may be aspirated with a syringe in some cases.

Undersaturation of aquarium water with oxygen may result in hypoxia and is fatal to fish unless corrective measures are taken. Fish experiencing this condition will commonly appear at the surface of the tank gasping for air. This behavior is called "piping" by aquarists.

As temperature increases the amount of dissolved oxygen decreases. It is interesting to note that water at 40 Centigrade contains twice as much oxygen as water at 400 Centigrade. Generally speaking, cold and cool water fishes require more oxygen than warm water fishes.

Two other factors to consider are pressure and salinity. As the salinity (salt concentration of the water) increases, the dissolved oxygen decreases. There is less oxygen present in sea water than in an equal volume of freshwater. As the atmospheric pressure decreases, the amount of dissolved oxygen also decreases. Thus, there is more oxygen present in a liter of water in Raleigh than there is in a liter of water in Denver with all other factors being equal. Aquatic plants and green algae also influence dissolved oxygen. During the day, photosynthesis occurs and aquatic plants give off oxygen. At night these same plants absorb oxygen and if numerous can deplete a body of water of available oxygen. This is rarely a problem in the home aquarium.

Tropical fish generally require between 6 and 10 parts per million dissolved oxygen to survive. Most fish become stressed below 6 parts per million. A helpful guideline is 6-8 liters of air per hour for every 3.8 liters (1 gallon) of water in the aquarium. At this rate there will be enough oxygen for the fish, plants and nitrifying bacteria. Most good pet stores will carry kits to test for dissolved oxygen if the clinician feels there may be an aeration or oxygen problem in the aquarium.

Many people speak of the importance of pH and its affect on fish health. The actual pH of the water is not nearly as important as how the pH is related to other water chemistry parameters such as ammonia. pH is simply a measurement of dissolved hydrogen ions in the water (the symbol stands for the logarithm of the reciprocal of the hydrogen ion concentration). Most fish can survive a wide range of pH values as long as changes which take place occur gradually. Marine fish are more sensitive to abrupt changes than freshwater fish and some species are more sensitive to these changes than others. When transferring fish from one aquatic system to another the pH values of both systems should be recorded. The normal acclimation process should include a "mixing" of the different water to allow for the fish to adjust to the change in water chemistry. This is especially important if the pH gap or difference is greater than 0.5. Changes of less than 0.5 are usually not too stressful for fish but normal mixing and temperature acclimation procedures should still be adhered too. If the pH gap is large (e.g. 6.0-8.5) then the mixing process should be in small volume increments and for an extended period of time (an hour or more if possible). Since pH is measured on a logarithmic scale, there are 100 times as many hydrogen ions in water with a pH of 6.0 as there are in water at a pH of 7.0.

In unbuffered aquatic systems there tends to be a gradual lowering of the pH since hydrogen ions are given off when ammonia is oxidized to nitrate by nitrifying bacteria. This is a common occurrence in systems which are consistently heavily loaded with fish. Most freshwater aquariums are best maintained at a pH of about 7.0 while marine tanks benefit from a pH of between 8.0 and 8.5. These values are only meant to be guidelines since certain species have their own unique requirements. There are a number of commercially available products which will both adjust the pH and buffer the water against pH changes. Sodium bicarbonate (baking soda) can be used to increase the pH of a tank while sodium biphosphate or small quantities of hydrochloric acid may be used to decrease the pH. Anytime one decides to adjust the pH of a tank with chemicals these compounds should be added carefully and gradually to avoid drastic pH changes. Experimenting first with water not containing fish or only a few fish is always a good idea when working with tropical fish. The buffering capacity of an aquarium will be discussed under the total alkalinity and hardness section. Aquariums which are not well buffered may require the addition of crushed coral or other limestone-like materials in order to increase the buffering capacity. These materials can commonly be added to the gravel at the bottom of the tank.

Although there is very little carbon dioxide present in atmospheric air there is a lot in natural water. After CO2 enters water, a small percentage is hydrated into carbonic acid. A portion of this carbonic acid then dissociates into carbonate and bicarbonate ions which are the primary buffers in freshwater.

Alkalinity can be defined as equivalent calcium carbonate and expresses buffering capacity. Don't confuse alkalinity with the term alkaline, a word used to describe water with a pH value greater than 7.O. While calcium carbonate is the primary buffer in water, borate (H2BO4) does account for about 5% of the total alkalinity in sea water. This buffering capacity is primarily dependent on the anions (bicarbonate and carbonate) and not on the cations (calcium and magnesium).

Hardness represents the total concentration of all cations in freshwater and is usually expressed in mg/liter calcium carbonate. Calcium and magnesium are the major cations associated with the carbonates (the source of alkalinity). The number of magnesium and calcium cations is frequently similar to the concentration of carbonate and bicarbonate anions. The term carbonate hardness is used to describe this condition. When alkalinity exceeds hardness some of the carbonate and bicarbonate is associated with sodium and/or potassium. When hardness exceeds alkalinity the calcium and magnesium ions are associated with anions other than carbonate and bicarbonate. In most cases freshwater hardness values are higher than alkalinity values.

Soft water (0-60 mg/L) generally has poor buffering capacity while hard water (greater than 180 mg/L) generally is a good buffer. If the hardness is high and the alkalinity is low than the previous statement will not be true. Alkalinity represents buffering capacity while hardness represents the dissolved cations. Measuring the dissolved cations in a water sample is usually an accurate but indirect way of determining buffering capacity. A good water quality test kit will be able to measure both total alkalinity and hardness.

Sea salt contains traces of many elements but 6 comprise over 99% of the mixture. These primary elements are: chlorine (Cl), magnesium (Mg), sodium (Na), calcium (Ca), potassium (K) and sulfur (S). Most ocean water is well mixed and rather stable in terms of salinity and elemental concentrations.

Chlorinity is defined as the amount of chloride ion (plus bromide and iodine) dissolved in 1 kilogram of sea water. By titrating chlorine and bromine with silver nitrate the chlorinity can be determined and then plugged into the following equation to obtain salinity:

  S(0/00) = 1.8 x Cl(0/00) + 0.03

  (0/00 = parts per thousand)

The simplest and safest way for the hobbyist to fill a marine tank is to use artificial marine mixes which simulate natural seawater. There are at least half a dozen of these mixtures on the market and the interested clinician should consult a reputable pet store dealing with marine tropicals for more information.

Next to oxygen, the nitrogen compounds which are dissolved in the aquarium water are the most important factors affecting the health of fish in the tank. Most nitrogen enters a tank in the form of fish nitrogenous waste. The cycling of nitrogen is performed by two genera of bacteria, Nitrosomonas and Nitrobacter. Recent research has shown that there may be other general of bacteria contributing to nitrification in aquatic systems. These bacteria and the substrate they adhere to is called the biological filter and will be discussed later. Ammonia, nitrite and nitrate are the nitrogen compounds of importance in an aquarium system.

Ammonia found in water is generally either in the toxic unionized form (NH3) or in the non-toxic ionized form (NH4+). The ratio between the two compounds depends on temperature, pressure, salinity and most importantly pH. The general rule is the higher the pH, the more unionized (harmful) ammonia present. The total ammonia reading represents both forms of ammonia combined. A total ammonia measurement of 3.0 ppm would be deadly at a pH of 8.5 in a freshwater tank but relatively harmless at a pH of 6.0 for a few days. Hobbyists and professionals alike commonly ask at what point should ammonia levels be considered dangerous? The best answer is that any detectable ammonia in an established aquarium is an indicator of a filtering deficiency. Either the filter is inadequate for the tank or the biological load is too great for the filter. An elevated ammonia level combined with a low pH may keep the fish alive but the ammonia problem itself needs to be tackled or fish disease will develop in time.

Nitrite is an intermediate compound in the nitrogen cycle and is converted to nitrate by a healthy biological filter. In a freshwater tank levels above 1.0 ppm will likely be harmful to the fish. As with terrestrial vertebrates, nitrite forms methemoglobin in the blood which results in respiratory compromise. Affected fish will show signs of oxygen deprivation by pumping their gills excessively and "piping" at the surface for air. Marine tropical fish are less sensitive to elevated nitrite since the abundant chloride in seawater competes with nitrite for uptake at the gill membrane.

Nitrate is the final nitrogen compound in the nitrogen cycle and is usually not toxic to fish but persistently high levels (over 50 ppm) is probably stressful to some species. Elevated levels in an aquarium may lead to excess algal growth in the tank. Regular water changes and nitrogen monitoring will help alleviate this problem.


All successful recirculating aquariums have at least one type of filter and many have two or more. A filter is just what it says it is; it filters or removes harmful or unwanted components from the water. Several different types of filters will be discussed.

The biological filter is the best and most efficient means of removing ammonia from an aquarium. This type of filter utilizes nitrification, a natural process which occurs constantly in soil and water. Nitrification involves the conversion of ammonia to nitrate in a two step process. Nitrosomonas oxidizes ammonia to nitrite and Nitrobacter oxidizes nitrite to nitrate. Stable populations of these bacteria must exist in the filter for the nitrification process to perform efficiently. There are likely even more nitrifying bacterial species than the two genera listed here. These bacteria require plenty of oxygen and ammonia as a food source. Under normal conditions it takes several weeks for the filter to develop and function adequately. When an aquarist attempts to "rush" the biological filter by loading a tank with fish before the filter is established he or she will likely be confronted with the "New Tank Syndrome." This syndrome is responsible for the death of countless tropical fish each year. Patience is the key when starting a new tank. Begin with a few hardy fish and then gradually add more fish over time when the filter becomes established. If the aquarist just has to have a full tank in the absolute shortest period of time then there are some commercially available biological filter starter solutions which contain nitrification bacterial cultures. Your local pet store merchant can help those interested in these products. The other alternative is some type of chemical filtration which physically removes the nitrogen compounds from the water. Sometimes a simple box filter containing activated carbon may be used to help remove some of the nitrogen load during the first few weeks that the tank is in operation. Care should be taken not to remove all of the available ammonia and nitrite or else there will be no "food" for the nitrifying bacteria.

There are several different types of biological filters. The first type is the popular undergravel filter. Most of these filters utilize a plastic grid which lies at the bottom of the tank allowing for a small water space beneath it. Several inches of gravel are then placed over the porous plate. Aerated water is pulled through the gravel bed via air lift tubes that are attached to the plastic grid. The bacteria become established on the gravel and the aquarium water is literally pulled through the filter bed exposing the nitrifying bacteria to the nitrogen compounds in the water. A second type of biological filter is termed the wet/dry filter and may also be called an ammonia tower or trickle filter. With these filters, the bacterial bed is not submerged in water but rather is sprayed with aerated water which passes through the filter bed by means of gravity. These filters are desirable since they tend to allow for a large surface area and consequently many bacteria can colonize the filter and much ammonia can be converted to nitrate. The large municipal sewage treatment plants operate with gigantic versions of the wet/dry biological filter. Many enterprising aquarists construct their own biological filters and those with less time or mechanical aptitude can buy wet/dry filter systems at the local pet shop or aquarium supply store.

A mechanical filter is any type of filtration apparatus which actually strains particulates from the water. These filters will usually not remove particles smaller than 3 microns and thus will not remove ions such as ammonia and nitrite. Submerged box filters, out of tank power filters, sand filters and diatom filters all utilize some form of mechanical filtration. Most good recirculating aquarium systems combine some type of mechanical filtration with the biological filter.

The use of activated carbon is the most popular means of chemical filtration used in the tropical fish hobby. Activated carbon binds organic compounds efficiently and may act as a substitute for a biological filter. When treating a tank with antibiotics or other therapeutic agents it is important to discontinue the carbon filter during treatment since the charcoal will remove the medication from the water (do not discontinue aeration however).

There are two other types of filtration that are gaining popularity in the tropical fish industry. These are the ultraviolet filter and ozone filter or ozonator. These two filters are commonly used together since ultraviolet light will inactivate ozone which can be harmful to aquatic life in the tank. These filters are not usually directly exposed to the aquarium water but rather some of the water from the aquarium is shunted to the filters so that over a period of time all of the water has passed through the filters. When water flows are properly controlled and the filters are adequately maintained these filters can remove organics from the water and kill both bacteria and some suspended parasites. Ozone and ultraviolet filters are very expensive and are not common in the home aquarium system.

Introduced Toxic Compounds

When evaluating an aquarium or a group of aquariums one should always find out the source of the tank water. If the water is from the tap then it should be determined whether it is city water or well water. Most municipal water has been chlorinated to sterilize it for safe human consumption. While relatively harmless to humans, chlorine can be deadly to tropical fish. The amount of chlorine in tap water may fluctuate but is usually between 0.5 and 2.0 mg/L. Chlorine can be "bubbled" out of water if the water is well aerated for several days in a container allowing for a large surface area. While effective, this method is time consuming and some people tend to lose track of time and may introduce fish to water that still contains some chlorine. There are a number of commercially available compounds which remove the chlorine from tap water instantly. These products usually contain sodium thiosulfate which inactivates chlorine through a chemical reaction in which sodium chloride is formed. Sodium thiosulfate is cheap, effective and safe to use. Just 7 grams of sodium thiosulfate will remove the chlorine from 1000 liters of municipal water (chlorine concentrations as high as 2.0 mg/L).

Another commonly used city water sterilizer is chloramine. This compound combines chlorine with ammonia, both of which are harmful to tropical fish. The reasons behind the use of chloramine instead of straight chlorine are centered around human health concerns. Simply bubbling water or letting it stand for a week or more will not remove chloramines. Water containing chloramines must be treated with a dechlorinator like sodium thiosulfate. Your municipal water supply office can provide the interested individual with more information on how the local water has been treated.

A municipality may occasionally add copper sulfate to the drinking water in low concentrations to help control an algae problem. Copper test kits are available in most pet stores. Levels over 0.15 ppm are dangerous to freshwater fish while levels exceeding 0.2 ppm can be harmful to marine fish. The lower the total alkalinity of the water, the more toxic the copper is. Invertebrate animals such as crabs, shrimp, anemones and snails are extremely sensitive to copper in the water. This fact must be kept in mind if copper is being used to control a parasite problem in the tank. If high copper concentrations are consistently found in the water a special copper filter may need to be employed to remove this harmful compound. Copper pipes in a building's plumbing may also be a source of copper in the water and there presence may have to be ruled out if there is a problem with copper in the water.

Temperature Control

Temperature is one of the easiest parameters to control in an aquarium yet it is commonly overlooked by the beginning hobbyist. Simply put most species of tropical fish require consistently warm water. As a rule, freshwater tropicals thrive between 75 and 80 degrees Fahrenheit and marine tropicals prefer slightly warmer temperatures (78-84 degrees Fahrenheit). Certain species of livebearers like guppies do well at room temperature and most goldfish prefer slightly cooler water (room temperature is adequate).

Suggested Reading

1.  Beleau, M.B. 1988. Evaluating Water Problems. Vet Clinics of North America, Small Anim. Pract. 18(2):293-304.

2.  Gross, M.G. 1976. Oceanography. Charles E. Merril Co., Columbus, OH, 138 pp.

3.  Hawkins, A.D. 1981. Aquarium Systems. Academic Press, New York, 452 pp.

4.  Noga, E.J. 1996. Fish Disease: Diagnosis and Treatment, Mosby-Yearbook, St. Louis, MO, 367 pp. (Reprinted in 2000 by Iowa State University Press).

5.  Schneider, E. 1982. All About Aquariums. T.F.H., Neptune, NJ, 127 pp.

6.  Spotte, S. 1992. Captive Seawater Fishes; Science and Technology. Wiley & Sons, 942 pp.

7.  Stoskopf, M.K. 1993. Fish Medicine. Saunders Co., 882 pp.

8.  Evans D.H. 1998. The Physiology of Fishes (ed.) 2nd. Ed. CRC Press, Boca Raton, FL.

*Modified and updated from an article which appeared in the Journal of Small Exotic Animal Medicine, 1991.

Back to Previous Page Print This Page Save This Page Bookmark This Page Go to the Top of the Page
Veterinarian Program
Veterinary Technician/Office Staff Program
Don J. Harris, DVM
Heidi Hoefer, DVM, Diplomate ABVP
David Holt, BVSc, Dip. ACVS
Debra F. Horwitz, DVM, DACVB
Amy Kapatkin, DVM, DipACVS
Karen Kline, DVM
Kenneth Kwochka, DVM, Diplomate ACVD Dermatology
Gregory A. Lewbart, MS, VMD, DACZM Aquatics/Reptiles
You are hereWater Quality and its Impact on Diseases of Fish
Diagnostic Techniques for Fish
Important Infectious Diseases of Ornamental Fish
Amphibian Medicine
Snake (Serpent) Medicine
Turtle (Chelonian) Medicine
Lizard (Saurian) Medicine
Reptile Formulary
Anesthesia, Analgesia, and Surgery in Pet Fish
Koi Medicine
Teresa L. Lightfoot, DVM Diplomate AABVP Avian
Howell P Little, DVM
Sandra Manfra Maretta, DVM
Wendy S. Myers
Karen Overall MA, VMD
Dr. Rodney L. Page & Dr. M. C. McEntee
Paul D. Pion, DVM, DipACVIM
Robert Poppenga, DVM, PhD
Karen Rosenthal, DVM, MS, ABVP
Howard B. Seim, III, DVM, DACVS
Robert G. Sherding, DVM, DACVIM Feline Medicine
Todd R. Tams, DVM
Brian T. Voynick DVM, CVA
Melissa Wallace, DVM, DACVIM Renal Medicine
Cynthia R. Wutchiett, CPA Management