Aquarium water parameters are the measurable properties of your tank water: temperature, pH, ammonia, nitrite, nitrate, hardness and, in saltwater, salinity plus the elements corals use. Each one tells you about a different part of the tank, such as how well waste is being processed, how steady the chemistry is, and whether the water suits the animals in it.
This guide explains what each parameter means. For target numbers, see our reef tank parameters chart and freshwater aquarium water parameters chart.
What are the main aquarium water parameters? #
| Parameter | What it tells you | Usual unit | Who tests it |
|---|---|---|---|
| Temperature | Metabolism, oxygen levels, heater health | °F or °C | Everyone |
| pH | How acidic or basic the water is | pH scale | Everyone |
| Ammonia | Waste that hasn’t been processed yet | ppm | Everyone, especially new tanks |
| Nitrite | Waste halfway through processing | ppm | Everyone, especially new tanks |
| Nitrate | The end product of processed waste | ppm | Everyone |
| KH / alkalinity | How well the water resists pH swings | dKH or meq/L | Everyone; reef keepers closely |
| GH | Dissolved calcium and magnesium in freshwater | dGH or ppm | Freshwater, especially shrimp keepers |
| Salinity | How much salt is dissolved | ppt or specific gravity | Saltwater and brackish |
| Calcium | A building block for coral skeletons | ppm | Reef |
| Magnesium | Keeps calcium and alkalinity in balance | ppm | Reef |
| Phosphate | A nutrient that feeds algae | ppm | Reef, and anyone fighting algae |
Which parameters show how waste is handled? #
Ammonia, nitrite and nitrate are three stages of the nitrogen cycle, and reading them together tells you how well the tank’s bacteria are keeping up.
Ammonia #
Fish excrete ammonia, and rotting food and dead animals release it. Test kits measure total ammonia, which exists in two forms: ammonium (NH4+), which is relatively harmless, and free ammonia (NH3), which is toxic. The University of Florida’s extension service explains that the toxic share rises as pH and temperature rise. So 0.5 ppm on your kit is far more dangerous in a warm tank at pH 8.2 than in a cool one at pH 6.8. In an established tank, any ammonia reading means something has gone wrong.
Nitrite #
Bacteria turn ammonia into nitrite, which is also toxic. In freshwater fish, nitrite gets into the blood and stops it carrying oxygen, a condition fish farmers call brown blood disease. Chloride in the water reduces how much nitrite fish absorb, as a Southern Regional Aquaculture Center fact sheet describes. Nitrite should read zero in a cycled tank.
Nitrate #
A second group of bacteria turns nitrite into nitrate, which is far less toxic and builds up over time. Water changes, plants and algae remove it. Nitrate is the best single measure of how your maintenance is keeping up with your fish load. Reef tanks keep it low because excess nutrients fuel algae and stress some corals.
Which parameters describe the water’s chemistry? #
pH #
pH measures acidity on a scale where 7 is neutral. Freshwater fish come from waters ranging from acidic blackwater to hard alkaline lakes, and reef tanks sit around 8. Stability usually matters more than hitting an exact number, because a fast swing stresses fish more than a steady value slightly off target.
KH and alkalinity #
KH (carbonate hardness) and alkalinity are close enough that hobbyists use the terms interchangeably. Both measure the water’s buffering capacity: its ability to absorb acid without the pH dropping. In freshwater, a very low KH means pH can crash. In reef tanks, corals use up alkalinity as they grow, so it drops daily and has to be replaced.
GH #
GH (general hardness) measures dissolved calcium and magnesium in freshwater. It matters for fish adapted to soft or hard water and for shrimp, which need minerals to molt. GH isn’t a useful measure in saltwater, where calcium and magnesium are tested separately.
Which parameters matter in saltwater and reef tanks? #
Salinity #
Salinity is how much salt is dissolved in the water. NOAA puts average ocean salinity at about 35 parts per thousand. Hobbyists measure it either in ppt with a refractometer or conductivity probe, or as specific gravity, where 35 ppt corresponds to roughly 1.026. Evaporation raises salinity, because water leaves and salt stays behind.
Calcium, alkalinity and magnesium #
Stony corals, clams and coralline algae build skeletons from calcium and carbonate, so they draw calcium and alkalinity down together. Magnesium doesn’t get used up much, but it stops calcium and carbonate from precipitating out of the water as they would otherwise. Reef keepers usually test these three as a set.
Phosphate #
Phosphate comes mainly from food. A little supports coral health, while too much fuels algae and can slow coral growth. Some kits report phosphorus instead of phosphate, which is a different number for the same thing.
Which parameters does each tank type need? #
| Tank type | Core parameters | Also worth testing |
|---|---|---|
| Freshwater community | Ammonia, nitrite, nitrate, pH, temperature | KH, GH |
| Planted freshwater | The above plus KH and GH | Iron, potassium, CO2 (via pH and KH) |
| Shrimp tank | Ammonia, nitrite, nitrate, GH, KH, temperature | TDS |
| Brackish | Salinity, ammonia, nitrite, nitrate, pH, temperature | KH |
| FOWLR (fish only with live rock) | Salinity, temperature, ammonia, nitrite, nitrate, pH | Alkalinity, phosphate |
| Reef | Salinity, temperature, alkalinity, calcium, magnesium, nitrate, phosphate, pH | Ammonia and nitrite in new tanks, trace elements |
Why do aquarium units get confusing? #
Several parameters are reported in more than one unit, and mixing them up causes most beginner mistakes:
- ppm and mg/L are the same thing in aquarium water.
- dKH and meq/L both measure alkalinity. Multiply meq/L by 2.8 to get dKH.
- ppt and specific gravity both measure salinity. 35 ppt is about 1.026.
- Phosphate vs phosphorus. Multiply phosphorus by 3.066 to get phosphate. Hanna’s ultra low range checker, for example, reads phosphorus in ppb.
- Nitrate vs nitrate-nitrogen. Some kits and most scientific papers report nitrate-nitrogen (NO3-N). Multiply by about 4.43 to get nitrate (NO3).
Why track water parameters over time? #
A single test is a snapshot. The useful information is usually in the trend: nitrate that doubles over a month, alkalinity that slides a little every day, pH that sinks each winter when windows stay shut.
Fish Tank Tracker logs ten of the parameters above (temperature, salinity, pH, ammonia, nitrite, nitrate, alkalinity, calcium, magnesium and phosphate) and charts each one over time with a shaded target band. There’s no GH field, so freshwater and shrimp keepers put GH in the notes, and the bands follow reef targets on every tank type. For consistent readings to put into any log, start with how to test aquarium water.
Frequently asked questions #
What are the most important water parameters for a beginner? #
Ammonia, nitrite, nitrate, pH and temperature. They tell you whether the tank has cycled, whether waste is building up, and whether conditions are steady. Saltwater beginners should add salinity from day one.
What does “stable parameters” mean? #
It means readings that stay close to the same values from test to test, without sudden jumps. Most fish and corals cope better with a steady value slightly outside the ideal than with a number that swings back and forth.
Can water parameters look fine while fish are sick? #
Yes. Standard tests don’t measure oxygen, disease, parasites, toxins from sprays or paint, or stress from tank mates. Good readings rule out the common water problems, but they don’t guarantee a healthy tank.
How often should I test my water? #
It depends on the tank’s age and type. New tanks need testing every day or two during cycling, established freshwater tanks usually every week or two, and reef tanks more often for alkalinity. Our guide on how often to test reef tank water goes into detail.