How (and How Often) to Test Aquarium Water for Healthy Fish and Plants

Regular water testing is vital for keeping tabs on the health of your aquarium fish and plants, but most beginners in the fishkeeping hobby are not aware of the importance of this practice. If the aquarium is “dirty-looking,” then people may clean the tank and do a water change. In reality, aquarium water contains invisible waste chemicals from the fish’s poop and other compounds that can be dangerous at high enough levels. Test kits are the only way to accurately measure if your water is clean and safe enough for fish and plants to live in.

How to Test Water in a Fish Tank

The most readily available types of water tests for fishkeepers are (1) test strips and (2) test kits that come with test tubes or other small containers. A chemical reagent is mixed with a sample of aquarium water and changes color based on the water parameter being measured. After a set amount of time, the reagent is compared to a color chart to tell you the final results. Here are the most common parameters we recommend looking at:

1. Ammonia: Ammonia is produced by your fish and invertebrates from their waste. It is very toxic to animals, especially in water with high pH, and should stay at 0 ppm (parts per million). Measure it with the Ammonia Test Strips.

Aquarium Co-Op Ammonia Test Strips

2. Nitrite: In a mature aquarium that is cycled, beneficial bacteria consumes the ammonia and produces nitrite. Nitrite is also toxic to animals and can burn fish gills and skin, so keep it at 0 ppm. Measure it with Multi-Test Strips.
3. Nitrate: In a mature aquarium, another type of beneficial bacteria consumes nitrite and produces nitrate, which is less toxic to fish. As a general rule, we recommend keeping nitrate at 50 ppm or below. If you have aquarium plants, they consume nitrate as food, so we like to maintain at least 20 ppm nitrate to keep them healthy. Measure it with Multi-Test Strips, and read our nitrate article to learn more.
4. Chlorine: If your drinking water comes from a municipal water supply, then most likely it is disinfected with chlorine or chloramine to eliminate pathogens. These same chemicals are lethal to animals, so a dechlorinator must be used to make the tap water safe. To make sure your chlorine is at 0 ppm, measure it with Multi-Test Strips.

Aquarium Co-Op Multi-Test Strips

5. pH: pH tells you how acidic or basic the water is. Most freshwater fish can live at pH levels between 6.5 to 8.0, but some species prefer lower or higher pH. Measure it with Multi-Test Strips.

6. GH: General hardness (GH) describes how hard or soft the water is, and it is measured in dGH (degrees of GH) or ppm. Minerals are essential for healthy animal and plant growth, so we recommend keeping between 4–8 dGH (or 70–140 ppm) for most freshwater aquariums. Measure it with Multi-Test Strips.
7. KH: Carbonate hardness (KH) measures the buffering capacity of the water. The higher the KH, the less likely the pH will rapidly change, which can be dangerous to fish. Like GH, it is measured in dKH (degrees of KH) or ppm, and we recommend keeping it at 3 dKH (50 ppm) or above for most freshwater aquariums to prevent pH swings. Measure it with Multi-Test Strips.

8. Phosphate: Phosphate is a macronutrient that plants need in order to grow well, but excess phosphate can cause algae growth and even harm fish health at high enough levels. Every aquarium has a different fish and plant stocking level, but as a general guideline, some hobbyists suggest 0.5–2 ppm phosphate for low light tanks and 3 ppm or more for high light aquariums that use CO₂ injection. 

API Phosphate Test Kit

9. Copper: Invertebrates are especially sensitive to even trace amounts of copper in the water, but some medications contain copper to treat certain fish diseases. Use a specialized copper test kit to measure the presence of copper in your tap water or to dose the correct amount of copper-based medication for sick fish.

API Copper Test Kit

10. CO₂: If you are setting up a DIY or pressurized CO₂ system, the Dennerle CO₂ Quick Test is an easy and accurate way to measure the dissolved CO₂ in your aquarium. Fill the prepared test tube halfway with tank water, shake for a few seconds, and immediately compare it to the color chart to see if you have too little, too much, or just the right amount of CO₂.

Dennerle CO₂ Quick Test

When and How Often to Test Aquarium Water

Ideally, water should be tested as often as possible, but in the past, test kits were often time-consuming and cost prohibitive to use very often. If fish keepers saw something odd in their tanks, they might ignore the problem and avoid testing the water because of these obstacles. Therefore, we developed the Aquarium Co-Op test strips to be faster and cheaper to use so that you can test more frequently for peace of mind. Here are the most common circumstances in which we recommend testing your water:

1. New Aquarium
   When setting up a new fish tank, it takes a while to cycle the aquarium so that the biological filtration is mature enough to purify the water from your fish’s toxic waste. While the aquarium is cycling, it is important to frequently test the water on a daily basis to make sure the ammonia, nitrite, and nitrate levels do not get too high, so get the Ammonia Test Strips and Multi-Test Strips. If the results are consistently safe and repeatable, you can decrease testing to every three days, then once a week, and eventually once a month. For more information on aquarium cycle, read our full article.

2. Tank Maintenance
   Once your aquarium is cycled, you may only need to use the Multi-Test Strips every 2–4 weeks to check the nitrate level, which can become toxic at very high levels. Generally speaking, we aim to keep nitrate at 50 ppm or below. If the nitrate test reads 75 or 100 ppm, than we know it’s time to do a water change. In fact, one of the reasons why we keep live plants in our aquariums is because they help consume nitrate and thus can minimize the number of water changes we need to do. Follow our water chart flow chart to figure out how often to do water changes based on the nitrate reading.

3. Sick Fish
   If your animals are displaying signs of illness or some are missing from the tank, it’s time to check every parameter possible to help you diagnose the issue. Start by checking the water temperature, Ammonia Test Strips, and Multi-Test Strips. If you suspect an abnormal increase in pH, use the API High Range pH Test Kit. Invertebrates like shrimp and snails are more susceptible to copper, so if you notice changes in their health, check your water with the API Copper Test Kit. The key is to not only determine if the measurements are within a healthy range but also to evaluate if they differ greatly from the usual values you have seen in the past.

Abnormal or rapid changes in water parameters can cause health issues in fish.

4. Unhealthy Plants
   When balancing the lighting and nutrients in a planted aquarium, nitrate is a key component to keep an eye on. Use the Multi-Test Strips to measure the nitrate level and keep it between 25–50 ppm. If nitrate is below this amount, then it may be time to dose some Easy Green all-in-one fertilizer to replenish the nutrients in the water. An overabundance or shortage in phosphate can cause problems like algae or leaves with large holes, so use the API Phosphate Test Kit to see what’s going on. Finally, if you are adding CO₂ gas to the water to increase plant growth, get a read on how much dissolved CO₂ is in the aquarium with the Dennerle CO₂ Quick Test.

5. Outdoor Pond
   For large outdoor ponds with huge volumes of water, we like to test at least three to four times a year using the Ammonia Test Strips and Multi-Test Strips. At the beginning of summer, we want to see how the water fared over the winter. In the middle of the summer, check the water quality because the fish have been eating different kinds of food and the pond evaporates faster in the warmer weather. At the end of pond season, make sure all the water parameters are safe before preparing for the cold weather. Finally, it may be good to do an extra test in the middle of winter to see how the fish are doing.

Water test kits can be used for both aquariums and outdoor ponds.

The more mature and problem-free a fish tank is, the less frequently we tend to test it, but don’t forget that your aquarium is a living ecosystem and things are constantly changing. If you go on vacation, change fish foods, buy or rehome fish, add or prune plants, or otherwise deviate from the norm, it’s best to test again and look for any anomalies. In order to keep track of water parameter values over time, many hobbyists mark them down in a journal or computer spreadsheet. For a fish room with multiple tanks, our CEO Cory will mark the results down on blue painter’s tape and stick it directly on the aquarium glass.

To learn more about water chemistry, we’ve gathered a series of articles to help you increase your knowledge and enjoyment of the fishkeeping hobby. Check them out and enjoy nature daily!