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Pet Owner Version

Providing a Home for Fish


Ruth Francis-Floyd

, DVM, DACZM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida;

Barbara D. Petty

, DVM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida

Reviewed/Revised Feb 2020 | Modified Oct 2022
Topic Resources

Once you have chosen the type of fish you want to keep, you need to learn how to set up and maintain the best possible environment to keep them healthy. Important considerations include water quality, diet, cleaning, and prevention and treatment of disease.

Set Up

Creating and maintaining a healthy and balanced aquarium or pond begins with the setup. A good setup is also easier to clean. (For more information on routine cleaning and maintenance, see Essential Maintenance Essential Maintenance Essential Maintenance .)


Your aquarium should be placed in a location that you can get to easily for feeding, cleaning, and general maintenance. The floor and the tank stand or cabinet must be strong, level, and stable. Because 1 gallon (3.8 L) of water weighs 8.3 lb (3.8 kg), even a small aquarium can be extremely heavy relative to its size.

The tank should be located out of direct sunlight. This helps limit the growth of algae in the tank. Although a moderate amount of algae is not harmful, a buildup of algae on the glass or in the water is unsightly and makes it harder to spot any health problems with the fish or their environment. In addition, the tank requires more cleaning. Direct sunlight can also cause the water temperature in the tank to rise. The warmer the water becomes, the less oxygen it will hold. If the water gets too warm, the fish can suffocate and die.

The same principles apply to outdoor ponds, although direct sunlight may be unavoidable. If the pond contains aquatic plants, which can help oxygenate the water or supplement the fish’s diet, some sunlight is necessary. Aquatic plants also help prevent the growth of algae by absorbing nutrients that would otherwise feed the algae. Plants, and floating plants in particular, can also minimize temperature changes by shading the water to keep it cool.

Another consideration in selecting a location is access to electrical outlets for the lights, filters, and pumps. Are there enough outlets close by or will you need extension cords and a power strip? Is the outlet grounded? Does it have a GFI (ground-fault interrupter)? In any case, you should consider a surge protector. The same is true for pumps, filters, and possibly lights operating for an outdoor pond. Outdoor plugs should be grounded and waterproof.

If you are installing an outdoor pond, either a preformed rigid liner or a flexible EPMD sheet liner can be used. Make sure the hole is deep enough for your purposes, free of any debris that might puncture the liner, and ideally lined with sand, both to act as a cushion and to help prevent frost heave (the expansion and contraction of the ground during freeze-thaw cycles).

Filtration, Heaters, and Lights

Filtration is important to remove waste and particulate matter. There are many kinds and types of filtration systems. The most common are under-gravel filters and hanging filters.

An under-gravel filtration system has a framework or grate that is placed underneath the gravel (or other substrate). Tubes are inserted into the framework through the gravel. Water is drawn up the tubes via either an external air pump or a submersible water pump (often called a power head). In an under-gravel system, the gravel acts as the filter medium, which traps particulate matter and contains nitrifying bacteria that remove the primary fish waste, ammonia.

Hanging filters have an intake tube that reaches into the water. A small electric water pump draws water in through the intake. The water then passes through a replaceable filter cartridge that consists of filter floss, which collects particulate matter, and activated carbon, which removes certain chemicals and minerals. Many hanging filters also have some form of medium to hold the nitrifying bacteria that remove fish waste.

A heater is necessary for both freshwater and saltwater tropical fish. Do not plug in the heater until the tank is full of water. Most heaters are in a glass tube that may shatter if not surrounded by water, which regulates the temperature and prevents the tube from overheating.

Fish need a day/night cycle for optimal health. Leaving the lights on 24 hours a day, 7 days a week stresses fish, leaving them more susceptible to disease.

Filters, pumps, heaters, and lights should not be turned on until after the tank is filled with water. Many electronics for aquariums and ponds, particularly submersible pumps, need the water to keep them cool and running safely. Certain fish, such as tangs, in saltwater aquariums are very sensitive to the extremely low charge that builds up in the water with the use of these electronics. This electrical charge can cause various health problems and stress. Use of a grounding probe is recommended to bleed off this charge.

In small aquariums, such as goldfish bowls and betta tanks, neither filtration nor a heater may be required. However, more frequent water changes are needed, with more water being changed out as well—perhaps as much as 50% per week. Goldfish and certain other temperate-water or cold-water fish do fine in water that is room temperature.

Gravel and Decorative Objects

Usually, gravel is placed in a tank at a proportion of 1 pound of gravel per 1 gallon of water. Before gravel is placed in the aquarium, it should be rinsed thoroughly to remove any accumulated dust. Dust can prematurely clog filters and damage water pumps. If using an under-gravel filter, pile the gravel higher at the back of the tank near the tubes so it can act as the filtration medium. Creating a slope to the gravel can also make the tank appear to be deeper front to back.

Larger decorative objects, such as large rocks, coral skeletons, or driftwood, are easier to place before the tank is filled with water because they can be moved much more easily. Decorative objects are not only aesthetically pleasing but also help “divide” the aquarium and allow fish to have their own territory. This can be very important for certain fish, such as freshwater cichlids and many saltwater reef fish.


Poor water quality is the most common cause of environmental-related diseases in fish. Water quality must be routinely monitored for levels of chlorine, pH ( see Water Water Water ), temperature, and salinity (level of salt). The water quality should always be tested before adding fish to the aquarium or pond to ensure that the water conditions are optimal. Your veterinarian or local pet store may offer this testing service, or you can purchase the testing equipment and do it yourself. Having your own testing kit also lets you include water quality testing as part of routine water changes and cleanings, including filter medium changes.

An aquarium can be filled with tap water, filtered water, or reverse-osmosis water. Obviously, tap water is the most convenient, the least expensive, and generally acceptable to use. Depending on its source (city or well), tap water will probably require treatment in one or several ways. Most city water is treated with chlorine or a combination of chlorine and ammonia (known as chloramine), both of which are toxic to fish and to the necessary bacteria that grow in your aquarium. Additives (dechlorinators) are available to remove both chlorine and chloramine from the water and should be used to treat any new water that goes in the aquarium. Dechlorinators are fast acting, readily available, and inexpensive. Dissolved minerals can also lead to excess algae growth.

Explaining pH

pH is a measurement of the acidity or alkalinity of a substance. It is measured on a scale of 0 to 14, with 0 being the most acidic and 14 being the most alkaline. A pH of 7 means the substance is neutral. The pH scale is logarithmic, meaning the difference between any 2 whole numbers is a multiple of 10. It is important to recognize that small numerical changes represent significant changes in pH. The pH level of your aquarium should be monitored regularly because most fish have a narrow range of pH in which they will thrive.

Basic (alkaline)


9.0 (100 times more alkaline than a pH of 7.0)


8.0 (10 times more alkaline than a pH of 7.0)


7.0 (Neutral)



6.0 (10 times more acidic than a pH of 7.0)


5.0 (100 times more acidic than a pH of 7.0)


The pH level of tap water varies greatly from well to well or city to city. The level can easily be checked with one of many easy, inexpensive, and readily available home test kits. You can also take a sample of your water to your veterinarian or pet store for a more thorough analysis and determination of other water treatments that may be needed.

Filtered water has generally been passed through media to remove particulate matter, over activated carbon to remove chlorine and other chemicals and minerals, and possibly through or over another medium that may remove other compounds. Reverse-osmosis water is processed similarly, but the water is under pressure and forced through an extremely fine membrane and a similar activated carbon membrane to remove even more compounds from the water than are removed during normal filtration.

For a saltwater aquarium, the proper amount of commercial aquarium salt can be added to the tank before or after the water. Another option is to mix the salt with the water in a bucket before adding it to the tank. Regardless of method, you should follow the package directions and use the proper amount of salt to reach the desired salinity. For a saltwater environment, this is usually 30 to 33 parts per thousand. Commercial salts provide not only salinity but also necessary minerals and elements that are not found in tap, filtered, or distilled water. Commercial salts also raise the pH of the water and help buffer it to keep the pH stable.

Beneficial Bacteria

Beneficial bacteria feed on various wastes in the aquarium or pond. These can be solid wastes, such as uneaten food, decaying plant matter, and fish feces, or chemical wastes that the fish excrete—primarily ammonia. Prolonged ammonia exposure is highly toxic to fish. The ammonia is consumed by one type of bacteria that produces nitrites as its waste. These nitrites are also toxic to fish in high quantities. Another type of bacteria then consumes the nitrites to produce nitrates. Nitrates are much less toxic. They tend to accumulate more in saltwater tanks, however. Fortunately, nitrates can be used by plants and algae as food, or they can be removed by water filtration.

These bacteria are common in the environment and will develop on their own in 4 to 6 weeks. Commercial products may speed up the process slightly, but these are not necessary for most home aquariums.

The bacterial colonies will generally grow to match the amount of available waste, but this takes time. Adding too many fish at once will overwhelm the available bacteria, and toxins will build up quickly. This is why it is best to add fish gradually, 1 or possibly 2 at a time, to allow the bacterial colonies to grow to a level that can balance the amount of waste. The disadvantage of gradual addition of fish is that any time new fish are added to an established aquarium, there is a risk of introducing disease.

The maximum number of fish a system can safely handle can be a function of the available area, such as in many freshwater environments, or it can be a function of volume, as recommended in saltwater aquariums. A good freshwater guideline is 1 inch of fish for every 12 square inches of surface area, or 1 centimeter of fish for every 36 square centimeters of surface area. The ratio of fish to area should be reduced in a cold-water environment, such as ponds and goldfish tanks, because the beneficial bacteria do not function as well at lower temperatures. A good saltwater guideline is 1 inch of fish for about every 5 gallons, or 1 centimeter of fish per 8 liters. Home saltwater aquariums hold fewer fish than freshwater aquariums of the same size because saltwater carries less dissolved oxygen than freshwater. Also, saltwater fish tend to be more sensitive to crowding, which can cause undue stress and lead to illness. Ultimately, the carrying capacity of your aquarium is based on the filtration capacity (which can be determined by testing water quality) and the territory available for the fish.

Adding Fish

From a water quality perspective, it can be good to add fish slowly to a new aquarium, 1 or 2 at a time over a period of several weeks. The problem with this approach is that there is significant risk of accidentally introducing disease if the new fish have not been carefully quarantined Quarantine Preventing disease is always preferable to treating it. In most cases, a comprehensive fish health management program should focus on water quality, nutrition, sanitation (maintenance and cleaning)... read more . An alternative strategy is to stock the new aquarium and monitor the water quality, especially ammonia and nitrite levels, on a daily basis. Rising levels of ammonia or nitrite can be controlled by frequent water changes. This approach is more work but can result in a stable and healthy group of fish.

When new fish are added to the aquarium, the current residents sometimes respond aggressively, chasing or attacking the new fish. Aggressiveness toward new fish is common among freshwater cichlids and other larger (more than 3 inches [7.6 cm]) freshwater species and, to a slightly lesser extent, among saltwater reef fish. This is particularly true when space and territory in the aquarium are at a premium. Aggressive behavior is stressful for all fish in the tank, so it is helpful to reduce aggression toward new fish by using the following techniques.

First, be sure to “float” the bag with the new fish in it to allow the temperature of the water in the bag to equalize with the temperature of the water in the tank or pond (about 20 to 30 minutes). Adding the fish directly, without a gradual equalization of temperature, can cause shock and stress. While the bag is floating, rearrange the decorative objects in the tank. This helps break up established territorial markers, which may reduce territorial aggression. This technique can also be helpful with aggression among established fish. When adding fish, never allow water from the transport bag to be introduced into the aquarium.

A second technique is to feed the fish in the tank at the same time the new fish are released. This is often enough to distract the aggressive fish from bothering the new fish.

A third technique is to release the new fish in the dark. When the tank light and the room lights are turned off, most fish will go into a “rest” mode within a few minutes. Float the bag with the new fish until the water temperature has acclimated and then release the new fish. After several minutes, turn the lights back on to feed or rearrange the decorative objects again if necessary.

If the aggression persists, the fish can be separated with a clear plastic tank divider. If this is not practical and other options have failed, it may be necessary to remove either the aggressive fish or the new additions.

Aggression among pond fish such as koi and comet goldfish is fairly rare, but the same techniques can be used.


Like all other animals, fish benefit from a well-balanced diet, which is essential for good health. Quality foods and variety also improve coloration and behavior and reduce stress levels.

You need to know the type of diet your fish require, as well as where your fish naturally feed in the water column, such as at the surface, in the middle, or at the bottom. For instance, fish that prefer to feed at the bottom should not be fed flakes that float on the surface. A wide variety of processed and natural foods are available to meet the needs of nearly any fish. Commercial dried foods, such as pellets, flakes, or granules, are a good choice because they are usually formulated to contain the proper mix of nutrients for your fish when fed properly. Food should not be stored for excessive periods of time. It is better to buy smaller quantities of food frequently than a large container that is used for many months. In general, dried foods should be kept on hand for no more than 2 months.

Feeding methods and rates vary with species, age, system, and water temperature. In general, maintenance diets are fed at a rate of 1% to 2% of body weight per day, and growth diets at 3% to 5% of body weight per day. Overfeeding can lead to health problems as well as a dirty tank. A variety of foods should be fed, with one primary dietary source supplemented with other foods that provide necessary nutrients.

Live foods, while generally a good source of protein and fat, can lack sufficient amounts of other elements and may not provide a complete diet. Another consideration is that fish fed only live food from day one may refuse other foods. If the primary live food becomes unavailable, it may take some time to find another source. Finally, live foods can sometimes introduce parasites or diseases into the fish’s environment.

Several kinds of pelleted food are formulated specifically for koi and goldfish in outdoor ponds. Natural foods such as shrimp pieces and plant material can be used as supplements. Insects that fall in the water are another source of live food in outdoor ponds. This amount should be reduced in colder weather because the fish are not as active and do not need as much food.

Fresh vegetables are also a good addition to the diet of many fish. Zucchini can be diced, lightly cooked, and stored in the freezer. Feeding vegetables a few times a week as a supplement provides an excellent source of B and C vitamins.

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