Aquarium Water Parameters: The Numbers That Actually Matter
Six numbers describe almost everything about your water. Three of them must stay at a fixed value forever; the other three just need to stay stable and suit your fish.
In This Guide
What to Test, and How Often
You need six readings. Three are pollution measurements that tell you whether the tank is safe. Three describe the water itself and tell you which fish belong in it.
| Reading | Target | How often |
|---|---|---|
| Ammonia (NH3) | 0 ppm | Weekly, and daily in a new tank |
| Nitrite (NO2-) | 0 ppm | Weekly, and daily in a new tank |
| Nitrate (NO3-) | Under 20-40 ppm | Weekly |
| pH | Stable, anywhere your fish tolerate | Weekly |
| GH (general hardness) | Matched to your species | Once, then after water source changes |
| KH (carbonate hardness) | 3+ dKH to hold pH steady | Monthly |
Temperature is the seventh, and the one you set rather than measure — see below.
Test strips give you the wrong answer
Strips drift badly with age and humidity, and they are least accurate exactly where it matters: near zero. A liquid reagent kit costs more once and lasts hundreds of tests.
Ammonia, Nitrite and Nitrate
Fish excrete ammonia, and uneaten food and dead plant matter release more. In a cycled tank, bacteria convert it in two steps.
Ammonia
Toxic at any detectable level. Burns gills and skin. Must read 0.
Nitrite
Also toxic — it stops blood carrying oxygen. Must read 0.
Nitrate
The safe end product. Removed by water changes and living plants.
Any reading above zero for ammonia or nitrite is an emergency: change 50% of the water with dechlorinated water matched for temperature, stop feeding for a day, and find the cause. Usually it is a tank stocked before it finished cycling, a filter that was cleaned too thoroughly, or a dead fish behind a decoration.
Nitrate is different. It accumulates steadily and is your cue for water changes. If nitrate sits above 40 ppm you are either overstocked, overfeeding, or changing water too rarely — in that order of likelihood.
Nitrate is also a tap-water problem
In some areas tap water arrives at 20-40 ppm nitrate on its own. Test your tap before blaming the tank; if it is high, water changes alone will not bring the number down.
pH: Stability Beats Perfection
pH measures how acidic or alkaline the water is, on a logarithmic scale — pH 6 is ten times more acidic than pH 7. Almost every commonly sold freshwater fish is farm-bred and adapts to a wide range, so the number itself matters far less than whether it moves.
A tank that sits at a steady pH 7.8 will keep fish that 'want' pH 6.5 perfectly healthy. A tank that swings between 6.5 and 7.8 every week will not.
- •Chasing a number with pH-adjusting chemicals is the most common way beginners cause a swing. Don't.
- •Low KH is the real culprit behind unstable pH. If KH is under about 3 dKH, the water has little buffering and pH will drop as the tank ages.
- •Driftwood, peat and botanicals lower pH slowly and gently; crushed coral and aragonite raise it. Both are safer than bottled adjusters.
The species with the narrowest tolerance in our database are the soft-water specialists: Chili Rasbora and Phoenix Rasbora go down to pH 4.0, while Peacock Cichlid starts at pH 7.8. Those two groups genuinely cannot share a tank.
Hardness (GH and KH)
GH is the amount of calcium and magnesium in the water. KH is the amount of carbonate, which resists pH change. Your water company publishes both, usually as 'water hardness' in mg/L CaCO3; divide by 17.9 to get degrees (dGH/dKH).
| Water | GH | Suits |
|---|---|---|
| Soft | 0-6 dGH | Tetras, rasboras, dwarf cichlids, most wild-caught species |
| Medium | 6-12 dGH | Almost everything sold in shops; the easiest range to keep |
| Hard | 12+ dGH | Livebearers, rift-lake cichlids, snails and shrimp |
Temperature
Temperature drives metabolism, oxygen levels and disease resistance. Warmer water holds less oxygen and speeds everything up, including the progress of an infection.
Most tropical community fish are comfortable at 24-26°C (75-79°F). The exceptions matter, because a heater set for one group is lethal over time for the other:
| Species | Minimum tank | Adult size | Temperature | pH | Group size |
|---|---|---|---|---|---|
| White Cloud Mountain Minnow | 10 gal / 38 L | 1.5 inches / 3.8 cm | 60-72°F / 16-22°C | pH 6.0-8.0 | 8+ |
| Peppered Corydoras | 15 gal / 57 L | 2.5 inches / 6.3 cm | 64-75°F / 18-24°C | pH 6.0-7.5 | 6+ |
| Hillstream Loach | 20 gal / 76 L | 3 inches / 7.6 cm | 68-75°F / 20-24°C | pH 6.5-7.5 | 3+ |
| Gold Barb | 20 gal / 76 L | 3 inches / 7.6 cm | 64-75°F / 18-24°C | pH 6.0-8.0 | 6+ |
| Zebra Danio | 10 gal / 38 L | 2 inches / 5.1 cm | 64-77°F / 18-25°C | pH 6.5-7.5 | 6+ |
| German Blue Ram | 20 gal / 76 L | 3 inches / 7.6 cm | 78-85°F / 26-29°C | pH 5.5-7.0 | — |
| Kuhli Loach | 15 gal / 57 L | 4 inches / 10 cm | 75-86°F / 24-30°C | pH 5.5-7.0 | 6+ |
| Discus | 55 gal / 208 L | 8 inches / 20 cm | 82-88°F / 28-31°C | pH 5.5-7.0 | 5+ |
The first five are subtropical and are slowly worn out by a heated tropical tank. The last three need the top of the range.
"Warm water fish" is a real incompatibility
A White Cloud Mountain Minnow tops out at 22°C (72°F). A German Blue Ram starts at 26°C (78°F). There is no temperature that suits both, however peaceful they are.
Match the Fish to Your Water
The shortcut that saves the most trouble: test your tap water once for GH, KH and pH, then choose species that already live in those conditions. Every species page on this site lists its temperature and pH range, and the browser lets you filter by them.
Test your tap, not your tank
Fill a glass, let it stand overnight to let gases equalise, then test pH, GH and KH.
Write down the range
For example: pH 7.6, 10 dGH, 6 dKH — moderately hard, slightly alkaline, well buffered.
Shortlist species that overlap it
Use the species browser and keep anything whose pH range includes your number.
Check the shortlist against each other
Run the combination through the compatibility checker before you buy.
Frequently Asked Questions
What are the ideal water parameters for a freshwater aquarium?
Ammonia 0 ppm, nitrite 0 ppm, nitrate under 20-40 ppm, KH at least 3 dKH, and a pH and temperature that stay stable within the range your species tolerate. Most community tropical fish are comfortable at 24-26°C (75-79°F).
Should I use chemicals to adjust my aquarium pH?
Usually not. pH adjusters wear off between doses and create the swings that actually harm fish. If your pH is unstable, raise KH instead; if it is simply not the number you wanted, choose fish that suit it.
How often should I test aquarium water?
Weekly for an established tank, and daily for ammonia and nitrite while a new tank is cycling or after adding a group of fish. Use a liquid reagent kit rather than test strips.
What nitrate level is too high?
Above 40 ppm is a sign the tank needs more frequent or larger water changes, less food, or fewer fish. Sensitive species such as discus and shrimp do better kept under 20 ppm.
Find fish that suit your water
Filter the species database by temperature, pH and tank size instead of guessing at the shop.