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Aquatic Water Quality and Care

Developed with and approved by a qualified veterinarian.

Freshwater Aquarium

Overview: What is water quality?

Water quality deals with the balance of bacteria levels, dissolved oxygen, contaminants, chemicals and minerals in your aquarium water. Good water quality is an essential factor in maintaining a healthy aquarium. Poor water quality stresses aquatic life, which leaves them susceptible to disease and possibly death. Below are some of the important components of water quality and how they work together.

What impacts aquarium water quality?


Water temperature is an important part of maintaining water quality and keeping fish healthy. Your aquarium’s water temperature should be within the appropriate range for the species you are caring for. Aquatic life can become stressed and susceptible to disease when exposed to a warmer or cooler temperature than they are accustomed to for an extended period, or when subjected to a temperature change greater than 2 degrees within a 24-hour period.

During power outages, an aquarium’s temperature can drop or increase rapidly. To retain heat, wrap the aquarium with a blanket; float bags of ice to safely reduce rising temperatures.


Ammonia (NH3) is produced when fish waste, organic matter and uneaten food break down. Ammonia is very toxic to aquatic life and is oxidized into nitrite by beneficial nitrifying Nitrosomonas bacteria.


Nitrite (NO2) is also very toxic to aquatic life, although slightly less harmful than ammonia. Nitrite is oxidized into nitrate by Nitrobacter bacteria.


Nitrate (NO3) is relatively harmless to aquatic life, unless aquatic life is exposed to high concentrations. In nature, plants use nitrate as fertilizer and complete the cycle by releasing oxygen into the water. Increasing nitrate levels are controlled with regular water changes. Plants utilize nitrates for proper growth and development, so live aquatic plants can be beneficial to the home aquarium. 


The pH scale measures pH levels from 1 to 14. The neutral pH level is 7, which means it is neither acid nor alkaline. As the scale goes down, it becomes more acidic, and as the scale goes up, it becomes more alkaline. The pH scale is logarithmic; every pH level is 10 times that of the previous level. Even a small change in pH levels on the scale represents a large change in water chemistry.

Your aquarium’s pH levels should coincide with the pH levels required by your aquatic life. Keep pH levels as constant as possible. Rapid changes in pH can cause stress and even death. Aquatic life can become stressed and susceptible to disease when exposed to a change in pH greater than 0.3 in a 24-hour period.

Recommended Salwater and Freshwater Aquarium pH Levels

Water hardness

Water hardness is the measurement of minerals—typically calcium, carbonate and magnesium—dissolved in a solution. Hard water has high levels of dissolved minerals and is usually high in pH. Soft water has low levels of dissolved minerals and is usually low in pH. In soft water, pH levels can change rapidly, but pH levels in hard water tend to be more stable.

If you need to change pH levels because you have extremely hard or soft water, Petco sells commercial products that aid in adjusting pH levels. Ask a Petco Pet Care Center partner for details.

Chlorine and chloramine

Municipalities add chlorine and chloramine to tap water to kill bacteria, but they are toxic to aquatic life. You can remove chlorine and chloramines from water with a standard chemical dechlorinator. Several chemical dechlorinators are available at your local Petco and online at


The copper levels in your existing tap water should be close to zero, which will not harm your aquatic life. However, in large amounts, copper is toxic to corals, plants and invertebrates and can be toxic to fish. If you live somewhere with high amounts of copper plumbing, be aware that copper could leach into the water supply. If you think you may have a concern with copper, buy a test kit. Chemical copper removers or reverse osmosis water filtration systems can remove copper, or you can find another water source for your aquarium.

Specific gravity

If you have a brackish or saltwater aquarium, it is important to maintain proper and stable specific gravity levels. Otherwise, your beautiful aquatic life will become stressed and susceptible to disease and death. Specific gravity is commonly measured with a hydrometer or refractometer. Check the specific gravity levels recommended for your species of aquatic life and maintain those levels. Specific gravity levels should not be allowed to fluctuate more than a level .001 within a 24-hour period. If utilizing freshwater aquarium salt, you should also test this level to ensure it remains stable between 1.003-1.004.


Alkalinity is water's ability to resist acidification. Alkalinity is important to the health of a reef aquarium. Alkalinity is a means of determining the amount of bicarbonate available in the water, often measured in dKH, and it should range between 8 and 12 dKH. These bicarbonates are one of the important components to building coral skeletons.


Calcium is another important mineral and building block for stony corals. Calcium is measured in parts per million. In nature, the world’s oceans typically range from 380 to 420 ppm. Maintaining calcium around 400 ppm is ideal for the home reef aquarium.


Magnesium is often overlooked but plays a crucial role in the chemical and biological makeup of a reef aquarium. Achieving balanced alkalinity and calcium levels can be difficult without a stable magnesium level. In nature, magnesium ranges from 1280 to 1350 parts per million; aim for similar levels at home. 


Phosphates are present in most water sources. In the home aquarium, phosphates can become elevated by uneaten foods, fish waste and decaying plant matter. Although phosphates do not directly harm aquatic life, elevated phosphates can pose a risk of algae blooms and stunted coral growth. Phosphates in the freshwater aquarium should try to be maintained at or below 0.2 ppm. To prevent issues in a reef aquarium, phosphate levels should try to be maintained at or below 0.03ppm. Phosphates should not be kept at zero because plants utilize them, and corals use small amounts for growth. 

Ongoing water care and maintenance 

Water test kit

Maintaining a healthy aquarium involves regular water quality checks. During the initial setup, water tests should be performed every couple of days. After ammonia and nitrite levels have reached zero and nitrate levels are within acceptable limits, water tests should be performed at least once a week. Poor water quality is the major cause of stress and disease in aquarium life. If any aquatic life looks stressed or ill, test water quality immediately.

Every aquarium should be tested for pH, ammonia, nitrite and nitrate levels. Brackish and saltwater need to be tested for specific gravity levels. The basic tests that should be included in test kits are:

  • Ammonia
  • pH
  • Nitrite
  • Nitrate
  • Specific gravity

For a reef aquarium, consider these additional tests:

  • Calcium
  • Alkalinity 
  • Magnesium
  • Phosphate

Note: Individual test kits are available that include instructions on how to test and adjust water quality. Additionally, Petco offers free water testing.

Biological filtration

Biological filtration occurs when nitrifying beneficial bacteria oxidize ammonia into nitrites and nitrites into nitrates. This process is called biological filtration because bacteria are required to complete this process. This process is also called the nitrogen cycle. When starting a new aquarium, ammonia and nitrites build up before stable colonies of beneficial bacteria develop and oxidize these toxins (sometimes called New Tank Syndrome).

Mechanical filtration

Mechanical filtration occurs when waste and debris suspended in the water are removed by passing them over materials that capture small particles, such as synthetic foam or nylon fiber floss. The mechanical filter can also serve as a home for beneficial bacteria, making it the medium for the aquarium's biological filtration as well.

Chemical filtration

Chemical filtration is the removal of dissolved substances from the water using absorbing media, such as carbon.

Supplies for caring for your water quality


  • How does water quality affect aquatic life? When water quality is poor or improper, it can cause stress, disease and even death in aquatic life.

Additional Care Sheets

Notes and resources

Ask a Pet Care Center store associate about Petco's selection of products available for the care and happiness of your new pet. All products carry a 100% money-back guarantee.

The information on this care sheet is not a substitute for veterinary care. If you need additional information, please contact your veterinarian as appropriate.