- Ammonia and nitrite should read zero in a cycled tank. Nitrate should stay low, which regular water changes handle.
- Stable beats perfect. Slow, steady conditions are healthier than chasing an ideal number.
- Match fish to the water you have, instead of changing your water to match the fish.
- When something is off, the first move is almost always a partial water change, then work out why.
What to test, at a glance
Water that looks clear can still be unsafe, and cloudy water can be perfectly fine. Testing is how you find out what is actually going on. This table is the quick reference. The sections below explain each line.
| Parameter | What it tells you | Typical goal |
|---|---|---|
| Ammonia | Fresh waste that the filter has not processed yet | 0 ppm in a cycled tank |
| Nitrite | A middle step in the cycle; toxic to fish | 0 ppm in a cycled tank |
| Nitrate | The end product of the cycle; builds up over time | Kept low, commonly under about 20 to 40 ppm; lower for sensitive species |
| pH | How acidic or alkaline the water is | Stable, and a fit for your species |
| KH | Buffering capacity that keeps pH steady | Enough to hold pH steady; many species like moderate KH |
| GH | Dissolved calcium and magnesium | Matched to your fish, plants, and shrimp |
| Temperature | Comfort and metabolism for your species | Within the species range, and stable |
| Chlorine and chloramine | Tap water disinfectants that harm fish and bacteria | 0 in the tank, neutralized with water conditioner |
Different kits report in different units. Ammonia, nitrite, and nitrate are usually in parts per million (ppm, the same as milligrams per liter). Hardness is often in degrees (dGH or dKH) or ppm. Check which units your kit uses before comparing numbers.
The nitrogen cycle in plain English
Fish eat, and what goes in comes out as waste. As food and waste break down, they release ammonia, which is toxic. A first group of beneficial bacteria converts ammonia into nitrite, which is also toxic. A second group converts nitrite into nitrate, which is far less harmful and removed by water changes and plant growth.
Those bacteria live mostly in your filter media and on surfaces in the tank. A new tank has few of them, so ammonia and nitrite spike until the colony grows. That process is called cycling, and it usually takes several weeks.
| Cycle stage | What your test shows | What it means |
|---|---|---|
| Early | Ammonia rising, nitrite and nitrate near zero | Bacteria colony just starting |
| Middle | Ammonia falling, nitrite rising | First group of bacteria working |
| Late | Ammonia and nitrite falling, nitrate rising | Second group working; almost there |
| Cycled | Ammonia 0, nitrite 0, nitrate present | Colony can process the waste in the tank |
If you are setting up a new tank, read our nitrogen cycle guide, which explains how to cycle with fish safely and why fish sometimes die in new tanks.
Ammonia
Healthy reading: 0 ppm. Ammonia burns gills and skin and stresses fish at low concentrations. How toxic it is depends on pH and temperature, because warmer, more alkaline water holds more of the toxic form.
Common causes
- A tank that has not finished cycling
- Overfeeding or uneaten food
- Too many fish for the filter
- A dead fish or plant rotting unnoticed
- A filter that was cleaned too hard, killing the bacteria
- Medications or large changes that harm the filter bacteria
- Do a significant partial water change with conditioned water at a matching temperature.
- Pause feeding for a day or so, and remove any dead or dying matter.
- Check that the filter is running and the media is not clogged.
- Retest within a day, and repeat water changes until it reads zero.
- Use a water conditioner that is labeled to handle ammonia as a short-term help, but fix the cause.
Nitrite
Healthy reading: 0 ppm. Nitrite interferes with a fish's ability to carry oxygen in its blood. Fish may gasp at the surface, hang near the filter outflow, or look sluggish. It is most common in new tanks and after the filter bacteria have been disturbed.
The fix mirrors ammonia: water changes now, cut feeding, protect your filter media, and give the bacteria time. Do not clean or replace all of your filter media at once.
Nitrate
Healthy reading: low. Nitrate is the least harmful of the three, but it builds up as fish produce waste. Many freshwater fish do well with nitrate kept under about 20 to 40 ppm. Sensitive species, shrimp, and some breeding setups prefer it lower. Planted tanks often keep some nitrate on purpose to feed plants.
How to bring nitrate down
- Do regular partial water changes. They are the standard way to keep nitrate in range.
- Feed what fish finish in a minute or two, and remove the rest.
- Keep a stocking level your filter and maintenance routine can support.
- Add live plants. They use nitrate as fertilizer, especially fast growers and floaters. See our planted tank guide.
- Check your tap water, because some tap supplies contain nitrate before it ever reaches your tank.
See also how often to do a water change.
pH, KH, and GH
These three get confused constantly, and hobbyists often change the wrong one. Here is how they fit together.
pH: acidic or alkaline
pH is a scale where 7 is neutral, lower numbers are more acidic, and higher numbers are more alkaline. Fish tolerate a range, and many adapt to what their tank holds. The danger is large or fast changes, not the exact number.
KH: the buffer that keeps pH steady
Carbonate hardness (KH) is the water's buffering capacity. Water with very low KH can swing in pH when acids build up, which sometimes shows up as a sudden drop, often called a pH crash. Water with moderate or high KH holds steady. If your pH keeps drifting, KH is the thing to check first.
GH: calcium and magnesium
General hardness (GH) measures dissolved calcium and magnesium. Shrimp need these minerals to molt properly, plants use them, and fish from hard-water regions expect them. Soft-water species, such as many tetras and rasboras, prefer lower GH.
Adding acid or buffer products to hit a target pH often creates swings that harm fish more than the original pH did. If your tap water is far from what a species needs, consider choosing a different species, or ask us about options before changing your water.
Matching fish to your water
These are general groupings. Individual species vary, and tank-raised fish are often more adaptable than wild-caught fish. Always research the exact species.
| Group | Often prefer | Examples |
|---|---|---|
| Soft, slightly acidic water | Lower pH, lower GH and KH | Many tetras and rasboras, rams, some dwarf cichlids |
| Neutral, moderate hardness | pH near neutral, moderate GH | Many corydoras, danios, gouramis, and community fish |
| Hard, alkaline water | Higher pH, higher GH and KH | Livebearers such as guppies, mollies, platies, and swordtails; African rift lake cichlids |
| Cooler water, wide tolerance | Moderate pH, steady quality | Goldfish, which also need strong filtration |
Our freshwater fish page and compatibility chart can help you plan.
Temperature
Temperature controls a fish's metabolism, immune function, and oxygen needs. Warmer water holds less oxygen and speeds up everything, from digestion to disease.
| Group | Typical range | Notes |
|---|---|---|
| Most tropical community fish | About 74 to 80 degrees F | Use a heater and a separate thermometer |
| Bettas | About 76 to 82 degrees F | Keep warm and stable; avoid drafts |
| Goldfish | About 65 to 74 degrees F | Generally do not need a heater in a heated home |
| Shrimp | Species dependent | Many common species prefer moderate temperatures |
Look up the exact species. See our heater page for help choosing equipment. A slow, small drift matters less than rapid swings, so avoid sudden changes during water changes by matching the temperature of the new water.
Tap water, chlorine, and chloramine
Most municipal water is disinfected with chlorine or chloramine, which harm fish gills and can kill your filter bacteria. Always treat tap water with a conditioner labeled to neutralize both. Chloramine is a combination of chlorine and ammonia, so conditioners that neutralize it can leave a little ammonia behind. That ammonia is normally handled by a healthy filter.
Your water utility publishes a water quality report each year. It is worth a look, because it often lists pH, hardness, and the disinfectant used. Better still, test your tap water in a glass for the same parameters you test in the tank, then test again after it sits for a day. Knowing your starting water makes every other decision easier.
How to test well
Pick a method
- Liquid test kits are the standard for hobbyists. They tend to be more accurate and economical per test, but you must follow the instructions exactly.
- Test strips are fast and convenient, and good for quick checks, though they can be less precise.
- Digital meters exist for pH, TDS, and temperature. They need calibration, and they are most useful for advanced setups.
Get consistent results
- Check the expiration date on your kit. Old reagents give wrong answers.
- Rinse the test tube with tank water before every test.
- Add the exact number of drops, holding bottles vertical, and shake or wait as the instructions say.
- Read the color under natural or neutral white light, against a white background.
- Write results down. Trends over time tell you more than any single number.
If a result surprises you, test again before acting, or bring a sample to us. A fresh sample in a clean, sealed container works best.
How often to test
| Situation | What to test | How often |
|---|---|---|
| Brand new tank, cycling | Ammonia, nitrite, nitrate | Every day or two |
| New livestock added | Ammonia and nitrite | Daily for the first week |
| Established, stable tank | Ammonia, nitrite, nitrate | Weekly or every other week |
| Planted tank with fertilizer | Nitrate, and phosphate if you track it | Every week or two |
| Fish look off, or a fish died | Everything | Right away |
| After medication, big change, or filter swap | Ammonia and nitrite | Daily until stable |
Symptom troubleshooting
Water tests do not explain every problem, but they are the quickest way to rule things out. Use this table as a starting point.
| What you notice | Check first | Common response |
|---|---|---|
| Fish gasping at the surface | Ammonia, nitrite, oxygen and surface movement, temperature | Water change, more surface agitation, find the cause |
| Fish hiding, clamped fins, no appetite | Ammonia, nitrite, temperature, new aggression | Water change and observe closely |
| Cloudy white water in a new tank | Cycle stage, overfeeding | Usually a bacterial bloom; avoid overfeeding and be patient |
| Green water | Light hours, nutrients | Cut light, do water changes, see our green water guide |
| pH keeps dropping | KH, nitrate, debris | Check KH; reduce waste and do regular water changes |
| Shrimp dying after a molt | GH, copper, sudden changes | Check hardness and any recent products; match water |
| Algae blooms | Light, nutrients, CO2 stability | Shorten light, clean up, review fertilizer |
| Plants melting or yellowing | Light, nutrients, new tank adjustment | See our planted tank guide |
For green water, read how to clear green water. For general tank health, see our common aquarium mistakes.
If several fish are in distress at once, do a significant water change right away, make sure the filter and heater are working, and then get help. Do not wait for another test result to act.
Common questions
What water parameters matter most for a freshwater aquarium?
Ammonia, nitrite, and nitrate matter most because they show whether your tank is cycled and safe. After that, temperature, pH, and hardness matter, mainly because stability and a good match to your species are what keep fish healthy.
What should ammonia and nitrite read in a healthy tank?
Both should read zero in an established, cycled aquarium. Any reading above zero means the tank needs attention, usually a water change and a review of feeding, stocking, and filtration.
What is a safe nitrate level?
Many freshwater fish do well with nitrate kept under about 20 to 40 ppm, with lower being better for sensitive species and for shrimp. Planted tanks often hold some nitrate on purpose to feed the plants. Regular partial water changes are the usual way to keep it in range.
Should I adjust my pH?
Usually not with chemicals. A stable pH matters more than a perfect one, and chasing a number with additives can cause swings that hurt fish. If your tap water pH suits a range of species, choose fish that match it. If you want to adjust pH, learn about your KH first, because KH is what keeps pH stable.
What is the difference between GH and KH?
General hardness (GH) measures dissolved calcium and magnesium, which matter for plants, shrimp, and fish health. Carbonate hardness (KH) measures buffering capacity, which is what resists changes in pH. They are related but not the same, and they change independently.
Why do I still get an ammonia reading after using water conditioner?
Some conditioners temporarily bind ammonia into a less toxic form, and some test kits still detect it. Others contain no binder at all, and chloramine in your tap water also releases ammonia when it is neutralized. If the reading stays above zero, keep testing, do water changes, and make sure your filter is cycled.
How often should I test my aquarium water?
In a new tank, test every day or two during the cycle. In an established tank, a weekly or every-other-week check of ammonia, nitrite, and nitrate is a good habit, and test any time fish look off, after changes to the tank, and after losing livestock.
Can I bring a water sample to the store?
Yes. Bring a fresh sample in a clean, sealed container, ideally collected the same day, and tell us your tank size and livestock. We will help you read the results and decide the next step.
Want a second set of eyes on your tank?
Bring a fresh water sample in a clean, sealed container, along with your tank size, livestock list, and photos if you have them. We will help you read the numbers and decide what to do next.