Every tropical fish on the market has a preferred range of water parameters. Get them right and the fish thrives, colors up, and breeds. Get them wrong and you have constant disease, refused food, and unexplained losses. This cheat sheet gives you the target ranges for the most common South American and tropical species, plus a working understanding of what pH, GH, KH, and TDS actually mean and how to adjust them.
The Parameters That Actually Matter
- pH — acidity/alkalinity on a 0-14 scale. Fish tolerate ranges, not points.
- GH (General Hardness) — total dissolved minerals (Ca, Mg). Measured in dGH (degrees) or ppm.
- KH (Carbonate Hardness) — bicarbonate buffering capacity. Stabilizes pH.
- TDS (Total Dissolved Solids) — all dissolved compounds. Measured in µS (microsiemens) or ppm.
- Temperature — mostly self-explanatory; affects metabolism and immunity.
- Ammonia (NH3) / Nitrite (NO2) — must be 0 ppm. Toxic at any level.
- Nitrate (NO3) — less toxic; under 20 ppm ideal, under 40 ppm acceptable.
Cheat Sheet: Common Species Parameters
| Species | Temp (°F) | pH | GH (dGH) | TDS (µS) |
|---|---|---|---|---|
| Wild Heckel Discus | 82-86 | 4.5-6.0 | 1-3 | 40-80 |
| Wild Brown / Royal Discus | 82-86 | 5.5-6.5 | 2-6 | 80-150 |
| Domestic Discus | 82-86 | 6.0-7.0 | 3-8 | 150-300 |
| Wild Pterophyllum scalare | 78-82 | 5.5-6.5 | 2-6 | 80-200 |
| Domestic Angelfish | 76-82 | 6.0-7.5 | 3-10 | 200-400 |
| Pterophyllum altum | 82-86 | 4.5-6.0 | 1-3 | 40-100 |
| Wild Apistogramma | 75-82 | 5.0-6.5 | 2-6 | 80-200 |
| Domestic A. cacatuoides | 75-82 | 6.0-7.5 | 3-12 | 150-400 |
| Cardinal Tetra | 76-82 | 4.5-6.5 | 1-4 | 50-150 |
| Rummynose Tetra | 76-82 | 5.5-7.0 | 2-8 | 100-300 |
| Neon Tetra (domestic) | 72-78 | 6.0-7.5 | 3-10 | 200-400 |
| Sterbai Corydoras | 78-84 | 6.0-7.5 | 3-10 | 150-400 |
| Most Corydoras | 72-78 | 6.0-7.5 | 3-10 | 150-400 |
| Ram Cichlid (M. ramirezi) | 80-86 | 5.5-7.0 | 3-8 | 100-300 |
| Bristlenose Pleco | 72-82 | 6.0-7.5 | 3-15 | 150-500 |
pH Explained
pH measures hydrogen ion concentration on a logarithmic scale 0-14. Below 7 is acidic, above 7 is alkaline, exactly 7 is neutral. Important: pH 5 is 100 times more acidic than pH 7, not “twice as acidic.” Each whole number step is a 10x change.
Most freshwater tropical fish tolerate pH ranges, not specific values. A discus rated “pH 6.5” will be perfectly healthy at pH 6.0-7.0. Stability matters more than the exact number — a stable pH 7.2 is better than a fluctuating “ideal” 6.5.
GH (General Hardness)
GH measures dissolved calcium and magnesium in your water — the “hardness” of tap water. Measured in dGH (German degrees) or ppm CaCO3 (1 dGH ≈ 17.86 ppm).
- 0-4 dGH (0-70 ppm) — very soft (blackwater / RO conditions)
- 4-8 dGH (70-140 ppm) — soft (most South American species)
- 8-12 dGH (140-210 ppm) — moderately hard (domestic strains, mollies, swordtails)
- 12+ dGH (210+ ppm) — hard (Rift Lake cichlids, livebearers)
Soft-water fish (discus, apistogramma, cardinal tetras) cannot breed and often cannot survive long-term in hard water. Their osmoregulation fails, leading to chronic kidney stress.
KH (Carbonate Hardness)
KH measures bicarbonate buffering capacity — the water’s ability to resist pH changes. High KH keeps pH stable but limits how acidic the water can become.
- 0-3 dKH — low buffer (blackwater); pH can swing easily
- 3-6 dKH — moderate buffer (most community tanks)
- 6+ dKH — high buffer; resists acidification (cichlid tanks)
Important: tannins and botanicals can only lower pH effectively in low-KH water. If KH is above 4-5, tannins barely affect pH no matter how many you add. This is why blackwater setups require RO water (zero KH) plus minimal remineralization.
TDS (Total Dissolved Solids)
TDS is measured with a cheap conductivity meter ($15-30) and gives you a single number summarizing all dissolved minerals and compounds. Useful as a quick check — if TDS is in range, your other parameters are usually within tolerance.
- 0-100 µS — RO water or very soft (Heckel discus territory)
- 100-200 µS — soft (wild aequifasciatus discus, wild Apistogramma)
- 200-400 µS — moderately soft (domestic strains, community tanks)
- 400-600 µS — moderate (livebearers, Rift Lakes)
- 600+ µS — hard (most municipal tap water in hard-water areas)
How to Test Your Water
- API Freshwater Master Test Kit — liquid drops, more accurate than strips. Tests pH, ammonia, nitrite, nitrate.
- API GH/KH Test — separate kit for hardness measurements.
- TDS meter (HM Digital, Apera) — instant conductivity reading.
- Avoid test strips — cheap but inaccurate, especially for pH and ammonia.
Test schedule: weekly for established tanks, daily during cycling, after every large water change.
How to Adjust Parameters
To lower pH and GH (for soft-water fish):
- Mix with RO water (50/50 to 80/20 RO/tap ratio)
- Add botanical tannins (Indian Almond, alder cones)
- Peat moss in filter (older method, less precise)
- Do NOT use chemical “pH down” products — they crash pH unpredictably
To raise pH and GH (for hard-water fish):
- Crushed coral or aragonite substrate (slow buffer)
- Seachem Equilibrium for GH
- Sodium bicarbonate (baking soda) for KH (use sparingly)
To stabilize parameters: do not chase exact numbers. A stable “wrong” value is better than a fluctuating “right” one. Pick the species first, then build the water to their needs.
Tap Water Reality Check
Before buying any fish, test your tap water. Most US municipal tap water reads:
- pH 7.0-8.5
- GH 6-15 dGH
- KH 3-10 dKH
- TDS 200-600 µS
Fish that work with this range without RO: most community tetras, livebearers, domestic angelfish, domestic discus (with care), Corydoras, plecos, ram cichlids. Fish that need RO + remineralization: wild discus, wild Apistogramma, wild angelfish from blackwater localities, P. altum, cardinal tetras for breeding.
Where Wild Fish Live (Wild Parameter Reference)
- Rio Negro (Brazilian Amazon) — pH 4.0-5.5, GH under 2, TDS 20-50 µS. Source of wild Heckel discus, Santa Isabel angelfish, cardinal tetras.
- Solimões / Amazon mainstem — pH 6.5-7.5, GH 2-5, TDS 80-200 µS. Wild aequifasciatus discus, many Apistogramma.
- Río Ucayali (Peru) — pH 5.5-7.0, GH 2-6, TDS 80-200 µS. Apistogramma cacatuoides, many Corydoras.
- Rio Orinoco (Colombia/Venezuela) — pH 4.5-5.5, GH 0-2, TDS 20-60 µS. P. altum, wild apistogramma.
Where to Buy Quality Tropical Fish
For wild fish that need specific water parameters, source from importers who quarantine and provide species-specific care info: Discus, Apistogramma, Angelfish from Amazonia Tropical Fish LLC, quarantined and shipped overnight with Live Arrival Guarantee.
For scientific background on water chemistry, Wikipedia’s water chemistry overview and hard water article are solid references.
Frequently Asked Questions
What is the most important parameter for tropical fish?
Stability beats any specific value. A tank held at stable pH 7.4 is healthier than one fluctuating between 6.5 and 7.5 daily. After stability: appropriate temperature, zero ammonia/nitrite, and species-matched GH/pH range.
Do I need RO water?
Test your tap water first. If tap reads pH 6.5-7.5 and GH under 10 dGH, you can keep many tropical species without RO. For wild blackwater fish (Heckel discus, P. altum, wild cardinal tetras), RO is essentially required.
Can I use API test strips?
Not recommended for pH or ammonia — strips give wildly inaccurate readings, especially in tannin-stained water. Use liquid drop tests for parameters that matter, supplement with a TDS meter.
Why does my pH keep crashing?
Low KH (carbonate hardness). Without bicarbonate buffer, biological acid production (from fish waste, decomposing food, botanicals) drives pH down. Either add crushed coral to stabilize KH at 3-4 dKH, or accept the low pH and use RO water for changes to prevent shock.
What is “good” tap water for tropical fish?
Slightly acidic to neutral (pH 6.5-7.5), GH 4-8 dGH, KH 2-5 dKH, TDS under 300 µS. This range works for most popular community species without modification.