Freshwater Fish In An Aquarium

If you care for aquarium fish properly, a disease rarely occurs. However, on those occasions when a fish does become ill, having information at your fingertips is critical. Here's your one-stop reference about fish health, disease treatment, and prevention. For community aquarium fish, here are the 10 basic rules to follow to keep your fish healthy: 

Water Testing

Before introducing any fish into your aquarium, buy yourself a good water test kit. Ideally, it should contain tests for ammonia, nitrate, nitrite, and pH. Test results, except for the pH, should all read zero before introducing any fish. The pH (acid-base balance) should be 7.0 to 7.8, depending on the species of fish being kept and your local tap water pH. Other water tests that are helpful to have are kits or test strips measuring the water hardness, alkalinity, and chlorine.


Make sure that the temperature in the aquarium is suitable for your chosen occupants. Use an aquarium heater and thermometer to keep the water at the correct temperature of the species of fish you keep.

  • Freshwater (generally): 69 F (21 C) – 80 F (27 C).
  • Marines (generally): 78 F (26 C).
  • Coldwater (generally): 56 F (13.5 C) – 68 F (20 C)


Filtration is the heart of any aquarium. It's the life source of the environment. There is a variety of filters available at pet stores, including undergravel filters using an air pump or electric water pump to move the water through it, hang on the back power filters, or canister filters that sit below the aquarium. The water flow through the filter should be about four times the water volume of the tank, so a 20-gallon tank should have a filter flow of 80 gallons per hour. The packaging on the filter system should indicate the water flow and the sizes of aquariums it is suitable for.

Stocking Levels

Overcrowding the aquarium with too many fish is one of the major causes of problems. Be sure you do not add too many fish for the size of the aquarium. Freshwater fish require a recommended 5 square inches (13 of surface area for each 1 inch (2.5 cm) body length of fully grown fish. For a fish only saltwater aquarium, the ratio is 1 inch (2.5 cm) of fish per 2 gallons (8 ltrs) of water and for reef aquariums, 1 inch (2.5 cm) of fish to 7 gallons (27 ltrs) of water. For outdoor ponds, allow 10 inches (25 cm) of fish to 132 gallons (500 ltrs) of water.


Learn as much as you can about each species you intend to keep. It is important to know how big the fish will be when it is full-grown, as most fish stores sell immature or young fish. Plan your stocking level based on the adult size of the fish, not the size it is when you purchase it. Also, research if the fish is compatible with other fish you already have; for example, large carnivorous fish would not be good to add in an aquarium with guppies or tetras or other small fish. While the water quality in the fish's natural habitat is important to know if the fish requires special water quality parameters, remember that most freshwater aquarium fish are raised in aquaculture farms now, and not collected from the wild, so the water they are bred and raised in may not be anything like the water they were originally native to.

Gradually Introduce Your Fish

Introduce fish into your newly set up aquarium gradually. Overloading your aquarium by adding all of the fish as soon as you set it up will cause problems. Remember, your filter will need to build up beneficial bacteria to break down the waste byproducts from your fish. It takes time for the good bacteria to grow, so add fish to new aquariums over 4 to 6 weeks after setting it up.

Quarantine New Fish

Do you really need to quarantine your new fish in a separate extra tank? The short answer is YES. Introducing new fish into your aquarium will invariably bring disease problems, so why risk it? Set up a small aquarium with a power filter and add some water from the main aquarium, then introduce the new fish in the quarantine tank for a week or so. You should monitor the water quality with your test kit and change the water as needed. It is also helpful to medicate the water to treat any common diseases, such as skin flukes or protozoan parasites. If there are no problems after a week or two, then move the fish into the main aquarium.


Twice daily is enough for feeding adult fish, and maybe more for young fish. Only feed as much food each time as will be eaten within 3 to 5 minutes, otherwise, excess food will decompose at the bottom of your aquarium, causing other problems such as high ammonia, phosphate and nitrate, and possibly unsightly algae growth.

Water Changes

Perform regular water changes, around 10-15 percent every 1-2 weeks. Use a gravel vacuum to remove the water and debris on the bottom of the aquarium, then refill with dechlorinated water at the correct temperature. It's good for your aquarium and your fish! Water changes remove ammonia, nitrite, and nitrate and replenish the alkalinity in the water, to help stabilize the pH.

Clean Filters

Clean your filters regularly according to the manufacturer's instructions. Avoid cleaning the sponge media in raw tap water; rather, use some of the water siphoned from the aquarium so the beneficial bacteria are not killed on the filter media and only the debris is cleaned away. If the aquarium filter has media that needs to be changed, don't change all of it at the same time so that the beneficial bacteria aren't depleted. Change only one media at each filter cleaning.

Health Basics

These basic resources will help you avoid serious health problems and recognize disorders when they do occur:

  • Flushing Fish
  • Using Salt
  • Signs of Fish Disease
  • Using Antibiotics
  • Medicated Foods
  • Malachite green and formalin


Disease treatment must be specific to the disorder. These disease profiles include steps for diagnosis, treatment, and tips for prevention:

  • Ammonia Poisoning
  • Columnaris
  • Cotton Mouth
  • Fin Rot
  • Flexibacter
  • Ich
  • Gold Dust
  • Nitrite Poisoning
  • Neon Pleistophora
  • Rust
  • Neon Tetra Disease

Article Sources

The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. Aquarium Water Quality: pH. Florida Department of Agriculture and Consumer Services.

  2. Introduction to Freshwater Fish Parasites. University of Florida Extension.

  3. Aquarium Care and Maintenance: Nutrition and Feeding. Florida Department of Agriculture and Consumer Services.

  4. Aquarium Water Quality: Total Alkalinity and Hardness. Florida Department of Agriculture and Consumer Services.

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