Water Parameters
Stable water is the foundation of a healthy aquarium. Use this cheat sheet as a starting point for typical community freshwater tanks and mixed reef marine tanks. Always match parameters to the specific livestock you keep, and make changes slowly.
Because freshwater and saltwater chemistry are completely different, the targets below are split into two sections. Test kits widely available in Australia (API, Red Sea, Hanna and similar) are ideal for checking these values at home.
Choose a topic:
Freshwater parameters Saltwater / marine parameters
Important: These are typical target ranges, not one-size-fits-all rules. Some fish and corals need different values. Test before you dose, change one thing at a time, and quarantine new livestock whenever possible.
Freshwater tank parameters
Target these levels. Keep your aquarium thriving.
| Parameter | Ideal range | Danger zone | Why it matters |
|---|---|---|---|
|
TemperatureTropical community
|
24 – 26°C (75 – 79°F) |
< 20°C | > 30°C |
Sudden swings stress fish and trigger disease. Keep heaters reliable and avoid placing tanks in direct sun or draughts.
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|
pHCommunity tanks
|
6.8 – 7.6 | < 6.0 | > 8.2 |
Affects how toxic ammonia is and how comfortable fish are. Stability matters more than chasing a perfect number.
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General hardnessGH
|
4 – 12 dGH (70 – 200 ppm) |
< 3 dGH | > 20 dGH |
Dissolved minerals that fish, shrimp and snails need. Soft-water species prefer the low end; livebearers and many Australian natives prefer harder water.
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|
Carbonate hardnessKH / alkalinity
|
3 – 8 dKH | < 2 dKH | > 12 dKH |
Buffers pH so it does not crash. Very low KH is common in RO or rainwater setups and needs careful management.
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AmmoniaNH3 / NH4+
|
0 ppm | Any detectable level |
Extremely toxic, especially at higher pH. A reading above zero usually means the tank is not cycled, is overstocked, or the filter has been disrupted.
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|
NitriteNO2
|
0 ppm | Any detectable level |
Very toxic to fish. Appears during cycling or after filter damage. Reduce feeding and do water changes until it returns to zero.
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NitrateNO3
|
5 – 20 ppm | > 40 ppm |
The end of the nitrogen cycle. Moderate levels are normal; high levels fuel algae and stress fish. Regular water changes and live plants keep it in check.
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PhosphatePO4
|
0.05 – 0.5 ppm | > 1.0 ppm |
Plants use some phosphate, but excess from food and tap water drives green water and hair algae. Reduce feeding and increase water changes if it climbs.
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Chlorine / chloramineTap water
|
0 ppm | Any detectable level |
Australian tap water is treated to kill bacteria — and it will also harm gills and filter bacteria. Always treat new water with a conditioner before it goes in the tank.
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Quick tips
- Test regularly, not only when something looks wrong
- Small, stable changes beat big swings
- Use a quality test kit and check expiry dates
- Water changes fix most nutrient problems
- Patience = a healthy tank
Testing frequency
- Weekly: pH, ammonia, nitrite, nitrate
- Fortnightly: GH, KH
- As needed: phosphate, chlorine after large water changes
- Ongoing: temperature (heater / thermometer)
Remember
Stable parameters create healthy fish, plants and invertebrates. Match the tank to the livestock — not the other way around.
Saltwater / marine tank parameters
Target these levels. Keep your reef thriving.
| Parameter | Ideal range | Danger zone | Why it matters |
|---|---|---|---|
|
SalinitySpecific gravity
|
1.025 – 1.026 (35 PPT) |
< 1.023 | > 1.027 |
Affects osmoregulation. Too low or too high stresses or kills livestock. Top up evaporation with RODI freshwater only, not saltwater.
|
|
TemperatureReef tanks
|
24 – 26°C (76 – 78°F) |
< 23°C | > 28°C |
Swings cause stress, bleaching and disease. A controller with heater and cooler (or a fan) keeps reefs safer through Australian summers.
|
|
pHSeawater scale
|
8.1 – 8.4 | < 7.8 | > 8.6 |
Impacts nutrient availability and overall tank stability. A night-time dip is normal; a crash usually points to low alkalinity or poor gas exchange.
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|
AlkalinityKH
|
7 – 11 dKH (2.5 – 4.0 meq/L) |
< 6 dKH | > 12 dKH |
Stabilises pH and is essential for coral skeleton growth. Keep it steady — daily swings are more harmful than a slightly “off” but stable number.
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CalciumCa
|
400 – 450 ppm | < 350 ppm | > 500 ppm |
Critical for coral skeletons and coralline algae. Dose two-part or use a calcium reactor once coral demand outpaces water changes.
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MagnesiumMg
|
1250 – 1350 ppm | < 1150 ppm | > 1450 ppm |
Helps keep calcium and alkalinity in solution. If calcium will not stay up, check magnesium first.
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NitrateNO3
|
1 – 10 ppm | < 1 ppm | > 20 ppm |
Balance is key: too high fuels algae and cyanobacteria; too low can leave corals pale. Export with water changes, a skimmer and macroalgae.
|
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PhosphatePO4
|
0.02 – 0.10 ppm | < 0.01 ppm | > 0.20 ppm |
Too high fuels algae; too low can starve corals. GFO, a refugium and careful feeding keep this in the sweet spot.
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AmmoniaNH3 / NH4+
|
0 ppm | Any detectable level |
Extremely toxic. Should always be 0 ppm in a cycled marine tank. If it appears, check livestock, feeding and whether the rock or filter has been disturbed.
|
|
NitriteNO2
|
0 ppm | Any detectable level |
Very toxic to fish and inverts. Should always be 0 ppm. A reading above zero is a cycling or biofilter problem — act with water changes and reduced feeding.
|
Quick tips
- Test regularly
- Small, stable changes beat big swings
- Use quality test kits (Red Sea, Hanna, API)
- Water changes fix many nutrient problems
- Patience = a happy reef
Testing frequency
- Daily: salinity, temperature, pH
- 2–3× per week: alkalinity
- Weekly: calcium, magnesium, nitrate, phosphate
- As needed: ammonia, nitrite
Remember
Stable parameters create thriving corals and healthy livestock. Consistency beats chasing perfect numbers.
