Aquarium Water Parameters to Control for Healthy Fish

Girl using a chemical test kit to measure the quality of water in a home aquarium
Elva Etienne / Getty Images

Aquarium water is the most important environmental component for your fish. Unfortunately, aquarium water quality is often overlooked by fish keepers, and sometimes neglected.

While humans can leave a smoky room or one filled with deadly car exhaust, fish are in a closed environment and can't escape if the water becomes toxic or dangerous.

Learn about water elements—like ammonia, nitrite, phosphate, and pH—that can cause harm for fish if not maintained properly and balanced carefully in an aquarium.

  • 01 of 10

    Ammonia Poisoning

    Ammonia is the natural waste product of fish metabolism and if it builds up in the water it is very harmful for the fish. Any time your fish are in distress or you have sudden fish death, consider increased ammonia as a possible cause.


    Ammonia poisoning is one of the biggest killers of aquarium fish. It occurs most often when an aquarium is newly set up, before the beneficial bacteria that break down fish waste have had a chance to grow, or "cycle." However, it can also occur in an established aquarium when too many new fish have been added at one time, when the filter fails due to power or mechanical failure, or if beneficial bacterial colonies die off due to the use of medications or a sudden change in water conditions.

  • 02 of 10

    Aquarium Algae

    Algae growth is a fact of life that every aquarium owner will face sooner or later. Some algae growth is normal and healthy, but excessive algae growth is unsightly and can be hazardous to fish and plants. Excess lighting, too much fish food, and lack of sufficient water changes can increase algae growth in your aquarium due to an accumulation of phosphate or nitrate in the water. If algae is a continuous problem, you might even want to consider adding an algae-eating fish, or using a commercial algaecide product made for aquariums.

  • 03 of 10

    Aquarium Water Testing

    Is aquarium water testing really necessary? Some fish hobbyists say categorically, no, while others test everything and anything. Water tests can be a great help to you if your aquarium is having a problem, but you are not sure of the cause. What should be tested, and how often, is not a simple answer—it is all dependent on your particular water quality and the problems you are experiencing. The basics include testing for ammonia, nitrite and nitrate. These are the waste components that fish produce and are harmful if they accumulate in the water. Minerals in the water can alter the acid-base balance (pH = power of Hydrogen), hardness (GH = General Hardness) and alkalinity (KH = Carbonate Hardness). Tests are also available for chlorine, chloramine, copper and phosphate, which can be found in tap water. Many types of test kits and test strips can be found at your local pet store, and in some stores they will test your water for free or at a low cost. In a new aquarium, the water should be tested daily or at least a few times per week. Once the aquarium is established and the water tests are normal, you can test the water every few weeks to monthly.

  • 04 of 10

    Cloudy Water

    Cloudy water can have several causes and depending on the cause, there is usually a corresponding cure. There is no magic bullet solution for cloudy water, it does take a little investigative work to come up with a solution. Based on the color of the water and the circumstances leading up to the appearance of cloudy water, you can usually find the root cause.

    In new aquariums, dust from the gravel, if it was not thoroughly rinsed before use, can turn the water cloudy. After a day or so in a new aquarium, bacterial blooms can also make the water appear cloudy, until the beneficial bacteria settle onto a surface to grow. If too much food is added to the aquarium, not only will the dissolving food make the water cloudy, but new bacteria growing to consume the extra nutrients will make the water cloudy.

    Using a water test kit to measure the ammonia and nitrite levels will help determine if they are high, which will cause bacteria to grow. Too much light, phosphate or nitrate can lead to green water: an explosion of algae growth. If the filter becomes too dirty it will lose its filtering capacity and the water may become cloudy. Water changes, cleaning the filter, increased filtration, and commercial chemicals added to precipitate suspended particles in the water will all help make the water clear again.

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  • 05 of 10

    Rocks That Can Affect Water Chemistry

    The use of rocks in your aquarium can affect water chemistry. It is often very difficult to know how and if a rock will affect your water. But, there are some ways to determine if the rocks you are about to use in your aquarium are safe or not. If adding vinegar or other acid on the surface of the rock causes any bubbling, it is best not to use it in the aquarium. You can soak rocks you want to use in a bucket of water and monitor the pH over a few days to see if there is any change caused by the minerals in the rocks.

    In freshwater aquariums, using gravel made from limestone, dolomite, aragonite, crushed coral or oyster shells will raise the hardness and pH of the water. It is better to use a quartz gravel for freshwater aquariums if the fish are not a species that requires the water to have a high pH (basic) or alkalinity. Always thoroughly rinse any rocks or gravel being use in a aquarium to remove any contaminants and dust.

  • 06 of 10

    Massive Water Changes Can Kill Fish

    Can water changes kill your fish? The quick answer to this is yes. Anything that suddenly changes the aquatic environment can kill your fish. The amount of water you change at one time and all the factors from temperature to pH and chemical composition to bacterial colonies may adversely affect the fish.

    Water changes are a must for a healthy aquarium, usually performed every week to once a month, depending on the aquarium conditions. So, when performing the water changes, be sure that the new water has been dechlorinated and is about the same temperature as the aquarium water.

    The pH of the new water should be adjusted to bring the existing aquarium water back to the correct level (usually 7.0-8.0, depending on fish species and local water pH), as the aquarium water pH gradually decreases (becomes acidic) over time and needs to be buffered (by increasing alkalinity) to bring it back to the correct level. Learn more about the safe ways to do water changes.

  • 07 of 10

    Nitrite Poisoning

    Nitrite poisoning follows closely on the heels of ammonia as a major killer of aquarium fish. Just when you think you are home free after losing half your fish to ammonia poisoning, the nitrite level rises and puts your fish at risk again. Anytime ammonia levels are elevated, increased nitrite will soon follow, and can quickly be lethal.


    Look for the warning signs of nitrite poisoning in your fish tank, such as poor appetite, inactivity, fish hanging out by the water filter outflow, and brown colored gills. Immediate water changes and adding 1-3 teaspoons of sea salt per gallon of water in the aquarium will help reduce the effects of nitrite toxicity.

  • 08 of 10

    Nitrogen Cycle

    The nitrogen cycle goes by many names: cycling, nitrification, the biological cycle, start-up cycle, and break-in cycle. No matter the term you use, every newly established aquarium goes through a process of establishing a beneficial bacterial colony. Older aquariums also go through periods during which the bacterial colonies fluctuate. Failure to understand this process is perhaps the largest contributing factor to the loss of fish. Learn about the nitrogen cycle, and how to deal with critical periods during the cycle.

    The first step of the nitrogen cycle is the production of ammonia by the fish and by bacteria that break down feces, uneaten food and other organic debris (detritus) in the aquarium. This ammonia is toxic to the fish. It takes time in a new aquarium for the beneficial bacteria to grow, so, if too many fish are added at once the ammonia can increase faster than the bacteria can break it down and the fish will die. Beneficial bacteria species will convert the ammonia into nitrite, but that is also toxic. After the ammonia is converted to nitrite, another species of bacteria starts growing that converts the toxic nitrite into non-toxic nitrate. The nitrate will accumulate in the aquarium water until it is removed by periodic partial water changes. This whole cycle can take 4-6 weeks to initially complete in new aquariums.

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  • 09 of 10

    Phosphate in the Aquarium

    Phosphate is present in every aquarium, even though many aquarium owners are not aware of it. Phosphate can be found in some cities' tap water, and is also in the food given to fish and can accumulate in the aquarium water. If the aquarium is not properly maintained, the phosphate levels will rise and contribute to algae growth. The results are not only unsightly but can become harmful to your fish.

    Contact your city water supply company to ask if the local tap water contains phosphate. You can also get a phosphate test kit to measure the phosphate level in the aquarium water. If the local city tap water is high in phosphate, performing water changes using tap water won't lower the phosphate in the aquarium. In this case, it is necessary to use deionized or reverse osmosis filtered water for your water changes. If the local water does not contain phosphate, then regular water changes using dechlorinated tap water can keep phosphate at low levels.

  • 10 of 10

    What is pH?

    Water pH measures how acidic or basic the water is. The term pH stands for the 'power of Hydrogen' and is measured on a scale of 1-14 units. Water is H2O, but it really is the ions Hydrogen (H+) and Hydroxyl (OH-) that make up water. If there is more H+ than OH-, then the water is acidic (pH 1.0 to 6.9). If there is less H+ than OH-, the water is basic (pH 7.1-14.0). When there are the same quantities of each the water is neutral and has a pH of 7.0. The 'H' in pH is always capitalized as H is the chemical symbol for Hydrogen.

    There is no one pH that is good for all fish. There are many different species of fish that live in different water environments, like the ocean, ponds, rivers and estuaries. Each of these bodies of water will have different pH levels. Saltwater fish may prefer a pH of 8 or higher, meanwhile freshwater fish may be more comfortable in a pH of 6 or 7. The best advice is to learn all you can about the species you plan to keep and attempt to mimic its natural habitat in your aquarium environment. However, most freshwater aquarium fish will do well in a pH of 7.0 to 7.5, as long as any change in the pH is done gradually over time.

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. Ammonia in Aquatic Systems. University of Florida Institute of Food and Agricultural Sciences.

  2. Cyanobacteria (Blue-Green Algae) Harmful Algal Blooms. The University of Tennessee Institute of Agriculture.

  3. Svobodova, Z., Machova, J., Poleszczuk, G., Huda, J., Hamackova, J., Kroupova, H. Nitrite Poisoning of Fish in Aquaculture Facilities with Water-Recirculating Systems. Acta Veterinaria Brno, 74, 129-137, 2005.

  4. Lingxiao, Ren, Wang, Peifang, Wang, Chao, Chen, Juan, Hou, Jun, Qian, Jin. Algal Growth and Utilization of Phosphorus Studied By Combined Mono-Culture and Co-Culture Experiments. Environmental Pollution, 220, 274-285, 2017, doi:10.1016/j.envpol.2016.09.061