"Water parameters" isn't a single thing. An aquarium is a living system with a dozen indicators that affect fish, plants and bacteria. The good news: you only need to actively track a handful — the rest takes care of itself as long as the tank is balanced.
This guide walks through every parameter that matters, explains what it means, what ranges to aim for, and what to do when something drifts. Treat it as a reference — come back to it when something's off.
1. How to measure parameters
Three kinds of tests
- Liquid reagent kits — JBL, Tetra, API, Sera. Most accurate for hobbyists, reasonably priced, slow to degrade. This is the standard.
- Test strips — cheap and fast for a quick screen, but inaccurate (especially after opening). Okay for rough checks; not for diagnosis.
- Electronic meters (pH, TDS, digital thermometer) — most accurate but need calibration. A pH meter is worth it on CO₂-injected planted tanks.
What and how often
- New tank (first 6 weeks): NH₃, NO₂⁻, NO₃⁻ every other day. This is the critical cycling window.
- Stable tank: NO₃⁻ weekly (before the water change), full panel every 2–4 weeks.
- When something's wrong: as often as possible. Every change — measure. Without data you're guessing.
2. Temperature
The most obvious parameter, yet often neglected. Fluctuations stress fish more than the absolute value — a steady 25 °C is better than a swinging 23–27 °C.
Typical ranges
- Tropical community (neons, corydoras, angels, tetras): 23–26 °C
- Lake Tanganyika and Malawi cichlids: 24–28 °C
- Discus: 28–30 °C
- "Cool-water" fish (goldfish, white clouds): 18–22 °C
How to control it
Heater sized at ~1 W per litre. A timer for a 1–2 °C nighttime drop is optional but mimics the natural day/night rhythm. In summer: a fan over the surface, or a sealed ice pack in extreme cases.
Consequences of getting it wrong
- Too cold: lethargy, appetite drops, higher disease risk (ich, fin rot).
- Too warm: less dissolved oxygen, stress, faster aging of fish.
3. pH
pH measures acidity on a 0–14 scale (7 = neutral). Different fish evolved in different water types — hence the wide range of "acceptable" values.
Typical ranges
- Most popular freshwater fish: pH 6.5–7.5
- Amazon species (neons, discus, apistos): 6.0–7.0
- Malawi and Tanganyika cichlids: 7.8–8.6
- Caridina shrimp (Crystal Red, Bee): 5.5–6.5
What drives pH
- KH (carbonate hardness) — the key buffer. Higher KH keeps pH stable.
- CO₂ — lowers pH. On CO₂-injected planted tanks expect 0.8–1.2 drop during the photoperiod.
- Peat, driftwood (mopani, mangrove) — lower pH via tannins and humic acids.
- Active substrates (ADA Amazonia, Tropica Soil) — lower pH by 0.5–1.0.
- Limestone, coral, dolomite — raise pH and KH.
Key rule:stability > value. Fish tolerate "non-ideal" but stable pH much better than daily swings.
4. Hardness — KH and GH
Two terms often confused, but very different. Together they describe the "chemistry" of the water that fish and plants feel.
KH — carbonate hardness
A measure of bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) content. Acts as a pH buffer — the higher the KH, the harder pH is to move.
- KH 0–2 °dKH — very soft, risk of "pH crash" (sudden drop at night or after dosing). Needs attention.
- KH 3–8 °dKH — typical for most freshwater aquariums. Safe.
- KH 10–20 °dKH — hard, natural for African rift lakes and spring water.
GH — general hardness
A measure of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. Affects fish osmoregulation, plant growth, and breeding success.
- GH 0–4 °dGH — very soft. Caridina shrimp, Amazon fish.
- GH 5–12 °dGH — typical. Most popular species.
- GH 15+ °dGH — hard. African cichlids, livebearers.
Adjusting hardness
- Lower: reverse osmosis (RO) + remineraliser, peat in the filter (slow).
- Raise: calcium-magnesium salt (e.g. GH+ Salty Shrimp), coral in the filter, dolomite in the substrate.
5. Nitrogen compounds — NH₃, NO₂⁻, NO₃⁻
Three parameters, one process — the nitrogen cycle. All three are worth measuring, especially in a new tank and when something's wrong.
How they behave over time
In a new tank ammonia peaks first, nitrite follows, and nitrate rises until the first water change.
Ammonia (NH₃/NH₄⁺)
Toxic. Below pH 7 the less harmful NH₄⁺ dominates; above pH 8 more of the dangerous NH₃.
- Target: 0 mg/L. Any reading > 0.25 mg/L is a problem.
- Sources: fish waste, rotting food, dead plants, decomposing biomass.
- What to do: immediate 50% water change, Seachem Prime (temporarily detoxifies), check the filter.
Nitrite (NO₂⁻)
Toxic — in some ranges more than ammonia. Blocks oxygen transport in fish blood (methemoglobinemia).
- Target: 0 mg/L.
- When it appears: in week 2 of cycling, or after filter "crash" (rinsing under the tap, long power outage).
- What to do: water changes + patience (bacteria need to rebuild). Bacterial starters can help.
Nitrate (NO₃⁻)
The end product of the cycle. Far less toxic, but accumulates over time and feeds algae.
- Community tank: 10–30 mg/L
- Shrimp and Amazon fish: 5–15 mg/L
- Planted tank (plants consume it): 15–25 mg/L
- Above 40 mg/L: time for a water change.
6. Phosphate (PO₄)
The "second macronutrient" alongside nitrate. Not toxic at typical concentrations, but excess — especially paired with high nitrate — feeds algae.
- Community tank: 0.1–0.5 mg/L
- Planted tank (plants actively consume it): 0.5–2 mg/L
- Above 3 mg/L — often correlates with algae outbreaks (especially black brush algae when NO₃ > 40).
Sources: fish food (especially frozen — shrimp, bloodworms), organic decay, some water conditioners.
7. Oxygen (O₂) and CO₂
Oxygen (O₂)
You rarely measure O₂ directly in freshwater tanks, but it's worth knowing the role it plays. Oxygen solubility drops with temperature — 26 °C has noticeably less O₂ than 22 °C.
- Signs of low O₂: fish near the surface, rapid gill movement, "gasping" in the morning before lights on.
- What to do: increase surface movement (raise the filter outlet or add an air stone), lower temperature, reduce CO₂ if dosing.
CO₂
Usually injected from a cylinder in planted tanks. Level dictates plant growth and fish safety.
- < 5 mg/L — plants struggle, algae outcompete plants.
- 10–20 mg/L — good range for moderately planted tanks without injection.
- 25–35 mg/L — optimal for lush injected planted tanks.
- > 45 mg/L — fish gasp. Reduce injection immediately.
CO₂ lowers pH — with injection, expect a 0.8–1.2 drop during the photoperiod. That's normal and safe as long as you're in the 25–35 mg/L zone.
8. Conductivity (TDS) and micronutrients
Conductivity (TDS)
A cumulative measure of all dissolved substances — salts, minerals, organics. Unit: μS/cm or ppm. Measured with a cheap electronic TDS meter.
- 50–150 μS/cm — very soft (RO + remineralisation). Caridina shrimp, discus.
- 200–400 μS/cm — typical European tap, medium hardness.
- 500–900 μS/cm — hard water. Livebearers, African cichlids.
Practical tip: rising TDS in a tank (at constant stock and dosing) means compounds are accumulating — usually nitrate. Time for a water change.
Micronutrients
Fe, Mn, Zn, Cu, B, Mo — needed by plants in trace amounts.
- Iron (Fe): 0.1–0.5 mg/L in planted tanks. Liquid tests exist but are inaccurate.
- The rest: dosed via "micro" fertilisers (Tropica Premium, Tropica Specialised, Easy Life Profito). Follow the label + watch plants.
- Copper (Cu): safe for plants when chelated, but toxic to shrimp above 0.03 mg/L. Watch out for medications containing copper.
9. Typical ranges by stocking
Quick reference for common setups. Values are midpoints of acceptable ranges — fine-tune per species.
| Stocking | Temp °C | pH | KH | GH | NO₃ mg/L |
|---|---|---|---|---|---|
| General community | 24–26 | 6.8–7.5 | 4–8 | 6–12 | 10–30 |
| Amazon (neons, discus) | 25–28 | 6.0–7.0 | 0–4 | 2–6 | 5–15 |
| Neocaridina shrimp | 22–24 | 6.8–7.5 | 3–6 | 6–10 | < 15 |
| Caridina shrimp (CRS) | 20–24 | 5.5–6.5 | 0–2 | 4–6 | < 10 |
| African rift cichlids | 24–27 | 7.8–8.6 | 10–18 | 10–20 | 10–25 |
| Livebearers (guppy, platy) | 23–26 | 7.2–8.0 | 8–15 | 10–18 | 10–25 |
| Angels, bettas, gouramis | 25–28 | 6.5–7.5 | 4–8 | 6–10 | 10–25 |
10. Quick troubleshooting
A short "how to tell that…" cheatsheet. Always start with a full parameter test — without measurements you're guessing.
- Fish at the surface, gasping → low oxygen, temperature too high, excess CO₂.
- Pale fish, loss of appetite, rapid breathing → ammonia or nitrite. Test immediately and water change.
- Older leaves yellowing → nitrogen deficiency (NO₃ < 5 mg/L).
- Holes in Anubias / Cryptocoryne leaves → potassium (K) deficiency.
- Black tufts on leaves and wood → black brush algae (BBA), usually from unstable CO₂.
- Cloudy white water after setup → bacterial bloom, normal, clears in 1–2 weeks.
- Cloudy green water → single-cell algae (Euglena), too much light + nitrate. Cut light, water change.
- White spots on fish → ich (Ichthyophthirius). Raise to 28–30 °C, add salt and medicine.
- pH swinging by 1+ unit daily → KH too low; buffer with dolomite or KHCO₃.
Wrap-up
You don't need to measure everything daily. In a stable tank, weekly NO₃ is enough (before the water change) plus a full panel every 2–4 weeks. But you do need to measure when something unusual happens — die-offs, algae, stressed fish.
The single rule of this guide: stability > ideal numbers. Fish evolved to tolerate imperfect environments, but they can't cope with constant changes. Keep parameters in a reasonable range and let the tank run.