Water chemistry underpins everything in a freshwater aquarium — from fish health and behaviour to plant growth and disease resistance. This guide covers every major parameter, explains why it matters, and gives you the ideal ranges for the most common freshwater setups we install and maintain.
pH measures how acidic or alkaline your water is on a scale of 0–14. A pH of 7.0 is neutral; below 7.0 is acidic, above 7.0 is alkaline. Most freshwater fish evolved in a specific pH range — being kept outside that range causes chronic stress, weakened immunity and shortened lifespan.
Ideal Ranges by Setup
| Setup Type | Ideal pH Range |
|---|---|
| Community freshwater (tetras, barbs, livebearers) | 6.8 – 7.5 |
| Planted tanks | 6.5 – 7.2 |
| Discus & soft-water species | 6.0 – 6.8 |
| African cichlids (Lake Malawi/Tanganyika) | 7.8 – 8.5 |
| South American cichlids | 6.2 – 7.2 |
⚠️ Rapid pH swings are more dangerous than a stable wrong pH. Stability matters as much as the number itself.
Ammonia is produced by fish waste, decaying food and plant matter. It is highly toxic — even at 0.25 ppm it damages fish gills and stresses the immune system. In an established, cycled aquarium, ammonia should always read 0 ppm.
Target: 0 ppm
If you detect any ammonia in an established tank: perform an immediate 25–30% water change, check for dead fish or uneaten food, and test again in 24 hours. If it persists, contact us — it signals a broken nitrogen cycle.
Nitrite is produced when beneficial Nitrosomonas bacteria convert ammonia. It is the second toxic stage of the nitrogen cycle — binding to fish haemoglobin and preventing oxygen transport (brown blood disease). Like ammonia, nitrite should always be 0 ppm in a cycled tank.
Target: 0 ppm
Nitrite spikes during new tank cycling (typically weeks 2–4) are normal. During this phase, avoid adding fish until both ammonia and nitrite have returned to 0 ppm and you register rising nitrate — confirming a full cycle.
Nitrate is the end product of the nitrogen cycle, produced when Nitrospira bacteria convert nitrite. It is far less toxic than ammonia or nitrite but causes chronic stress, depressed immune function and algae problems at elevated levels.
Ideal Ranges
| Setup Type | Target Nitrate |
|---|---|
| Community tanks | < 40 ppm |
| Planted tanks (plants consume nitrate) | < 20 ppm |
| Discus & sensitive species | < 10 ppm |
| African cichlids (more tolerant) | < 50 ppm |
Nitrate is managed through regular partial water changes — typically 15–25% weekly or 25–30% fortnightly.
GH measures the total dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) in your water. Fish use these minerals for bone density, osmoregulation and enzyme function. Soft-water fish (discus, tetras, apistogramma) thrive in low GH; hard-water fish (African cichlids, livebearers, goldfish) need higher GH.
| Water Type | GH (°dH) | Suitable For |
|---|---|---|
| Very soft | 0 – 4 | Discus, wild-type tetras, soft-water apistogramma |
| Soft | 4 – 8 | Most tetras, rasboras, corydoras, planted tanks |
| Medium | 8 – 12 | General community, barbs, most livebearers |
| Hard | 12 – 18 | Goldfish, Central American cichlids, livebearers |
| Very hard | 18+ | African cichlids, brackish species |
KH (Alkalinity or Carbonate Hardness) measures bicarbonate and carbonate ions. Its most important role is buffering pH — preventing dangerous swings. Low KH tanks are prone to sudden pH crashes, especially overnight in planted tanks when CO₂ is elevated.
| KH (°dH) | pH Stability | Notes |
|---|---|---|
| 0 – 2 | Very unstable | Risk of pH crash; soft-water specialist setups only |
| 3 – 6 | Moderate | Suitable for planted tanks with careful CO₂ management |
| 7 – 10 | Stable | Ideal for most community tanks |
| 10+ | Very stable | African cichlid and hard-water setups |
Temperature affects fish metabolism, oxygen solubility, disease susceptibility and biological filtration rates. Warmer water holds less dissolved oxygen — a critical consideration for discus tanks and heavily stocked setups.
| Setup Type | Temperature Range |
|---|---|
| Tropical community | 24 – 27°C |
| Planted tanks | 22 – 26°C |
| Discus | 28 – 30°C |
| African cichlids | 24 – 27°C |
| Goldfish (coldwater) | 10 – 22°C |
How to Test Your Water
We recommend liquid test kits over dip strips — they are significantly more accurate. The API Freshwater Master Test Kit covers pH, ammonia, nitrite and nitrate and is our go-to recommendation for home testing. For GH and KH, the JBL GH/KH test kit is reliable and easy to use.
Test frequency recommendations:
- New tank (weeks 1–8): Every 2–3 days for ammonia, nitrite and nitrate
- Established tank: Weekly pH and nitrate; monthly full parameter test
- After any change (new fish, medication, water source change): full test within 24 hours
Need a professional water test?
All our maintenance plans include full parameter testing on every visit, with a written report. Or book a one-off health check with Hasan.
Book a Water Test