Aquarium Water Parameters: Ideal Ranges and How to Test

Aquarium water can be crystal clear and still be toxic. Fish are immersed in it around the clock, so their health depends on things you can't see: ammonia, nitrite, nitrate, pH, hardness and temperature, among others. This guide explains what each parameter means, the ideal ranges for a tropical freshwater tank, how to test them and what makes them go up or down.

Ideal ranges at a glance

These ranges suit a typical tropical community tank. Some species have different needs, noted in the last column and on each fish profile.

Parameter	Ideal value	Notes
Ammonia (NH3/NH4)	0 ppm	Any reading means there is a problem
Nitrite (NO2)	0 ppm	Highly toxic even at low levels
Nitrate (NO3)	< 25-40 ppm	< 20 ppm for shrimp and sensitive species
pH	6.5-7.5	6-7 for soft-water fish; 7.8-8.5 for Lake Malawi and Tanganyika cichlids
KH (carbonate hardness)	3-8 dKH (54-143 ppm)	Below 2-3 dKH, pH becomes unstable
GH (general hardness)	6-12 dGH (107-214 ppm)	4-8 dGH for soft water; 12-20 for livebearers and African cichlids
Temperature	75-80 °F (24-27 °C)	Coldwater fish (goldfish, medaka) at room temperature
TDS (total dissolved solids)	150-350 ppm	Most useful in shrimp tanks

Ammonia, nitrite and nitrate: the nitrogen cycle

These three compounds are the stages of the nitrogen cycle. Fish excrete ammonia through their gills, and uneaten food and organic waste release it as they break down. In a cycled tank, one group of filter bacteria converts ammonia into nitrite and another converts nitrite into nitrate. That is why a new tank needs 4 to 6 weeks of cycling before you add fish.

Ammonia

It burns gills and skin. Its toxicity rises with pH and temperature: above pH 7, more of it exists as free ammonia (NH3), the most dangerous form. Overstocking, overfeeding, a dead fish you haven't spotted or an over-cleaned filter all push it up.

Nitrite

It stops blood from carrying oxygen, which is why poisoned fish gasp at the surface even with good aeration. It shows up during cycling and whenever something damages the bacteria colonies, such as medication or rinsing filter media under the tap.

Nitrate

It is the end product and builds up slowly. Only water changes and, partly, fast-growing plants bring it down. Long-term high levels shorten fish lifespans and fuel algae. Our guide to nitrite and nitrate covers it in depth.

pH, KH and GH: acidity and hardness

pH

It measures how acidic or alkaline the water is on a 0-14 scale, where 7 is neutral. Each step is a tenfold change, so going from 7 to 6 is a much bigger shift than it looks. Most store-bought fish adapt well to pH 6.5-7.5. Soft-water species such as discus prefer 6-7, while African cichlids such as the yellow lab need 7.8-8.5. If you have to adjust it, do it slowly, as we explain in how to raise or lower aquarium pH.

KH (carbonate hardness)

This is the water's ability to buffer pH changes. With KH at 3-8 dKH, pH stays stable; below 2-3 dKH it can crash suddenly, especially in tanks with a heavy organic load or injected CO2. Regular water changes replenish the KH that gets used up. In the US, test kits often read in ppm or drops: 1 dKH is about 17.9 ppm.

GH (general hardness)

It tells you how much calcium and magnesium is dissolved. It affects fish osmoregulation, shrimp and snail molting and plant health. Livebearers (guppies, mollies, platies) like hard water; tetras, rasboras and discus prefer soft water.

Temperature and TDS

Temperature drives fish metabolism, dissolved oxygen and ammonia toxicity. For most tropical species, the range is 75-80 °F (24-27 °C). More important than the exact number is that it doesn't swing more than 2-3 °F (1-2 °C) during the day. A thermostatic heater (rule of thumb: about 4 watts per gallon, or 1 W per liter) and an accurate thermometer are essential. There is more in our guide to aquarium temperature.

TDS (total dissolved solids) measures everything dissolved in the water in ppm: minerals, salts, nitrate, organics. It tells you how much, not what, but it is very handy for keeping shrimp tanks consistent and for checking that your RO water is clean. If TDS climbs a lot between water changes, substances are building up and you should change more water.

How to test your water

There are three types of test, each with pros and cons:

• Test strips: quick and convenient, they read several parameters at once. They are less accurate and degrade quickly once the bottle is opened. Fine for spot checks.
• Liquid test kits (drops): more accurate and cheaper per test. Master kits from brands such as API, Tetra, JBL or Sera cover ammonia, nitrite, nitrate, pH, KH and GH. This is the recommended option.
• Digital meters: for pH, TDS and temperature. Very handy, but pH pens need regular calibration.

You can compare options in our roundup of aquarium test kits. To get reliable results:

1. Take the sample from mid-water, away from the filter outlet, and before your weekly water change.
2. Rinse the test tube with tank water and follow the instructions exactly: number of drops, order and waiting time.
3. Shake the reagents that call for it (the nitrate bottle usually does, vigorously).
4. Read the color in daylight against a white background.
5. Write the results down with the date. Trends tell you more than a single reading.

In a new tank, test ammonia, nitrite and nitrate every 2-3 days while cycling. In an established tank, once a week or every two weeks is enough, plus any time your fish seem off.

Common water parameter mistakes

• Chasing a specific pH with chemicals: it causes swings that stress fish more than a slightly off pH ever would.
• Using expired tests: reagents have an expiration date and give false readings past it.
• Testing only pH: pH tells you nothing about ammonia or nitrite, which are what kill fish.
• Testing right after a water change: you will get better numbers than the real ones.
• Choosing fish without checking your tap water: it is far easier to pick species suited to your water than to change it.
• Assuming clear water is healthy water: ammonia and nitrite are invisible.

What affects each parameter

• Ammonia rises: overfeeding, overstocking, dead fish or plants, an undersized or freshly cleaned filter.
• Nitrate rises: too few water changes, heavy feeding, few or no plants.
• pH drops: low KH, driftwood and leaves (tannins), injected CO2, organic buildup.
• pH and hardness rise: limestone rocks, shells, crushed coral, hard tap water.
• TDS rises: topping off without water changes, fertilizers and additives, shrimp mineral salts.

Knowing your parameters lets you get ahead of problems instead of reacting once fish are already sick. And when something goes wrong, the first fix is almost always the same: a good water change.

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