KH and GH for discus: water hardness (anti pH crash)
KH stabilizes pH (anti pH crash); GH supports the fish's osmoregulation. For discus: 2–5 dKH and 3–8 dGH. How to measure and adjust with baking soda or Seachem Equilibrium.
Quick summary
- Ideal KH (carbonate hardness): 2–5 dKH — stabilizes pH.
- Ideal GH (general hardness): 3–8 dGH — healthy osmoregulation.
- KH under 1 dKH = a guaranteed overnight pH crash.
- Raise KH with baking soda (0.5g/10L = +1 dKH).
- Raise GH with Seachem Equilibrium (1g/10L = +3 dGH).
- Change gradually, max ±1 dKH/day, to avoid stress.
In short
- KH (carbonate hardness): 2–5 dKH — buffers pH against swings.
- GH (general hardness): 3–8 dGH — calcium and magnesium for osmoregulation.
- KH under 1 dKH = pH crash risk (the most dangerous situation).
- How to raise KH: baking soda (NaHCO₃) — 0.5 g / 10 L = +1 dKH.
- How to raise GH: Seachem Equilibrium — 1 g / 10 L = +3 dGH.
- Pure RO water = KH 0, GH 0 → remineralization is mandatory.
What you'll learn
KH and GH are two different measures of water hardness, often confused:
- KH = the water's ability to resist pH changes (buffering)
- GH = dissolved calcium + magnesium (for the fish's biological functions)
This guide clarifies the difference, the target values for discus, and how to avoid a pH crash (the most serious beginner mistake). For the full parameter table, see Ideal water parameters.
KH vs GH — the difference
| Aspect | KH | GH |
|---|---|---|
| Stands for | Carbonate Hardness | General Hardness |
| Measures | Carbonates + bicarbonates | Calcium + magnesium |
| Effect on fish | Indirect (pH stability) | Direct (osmoregulation) |
| Unit | dKH or ppm CaCO₃ | dGH or ppm CaCO₃ |
| 1 degree = | 17.9 ppm CaCO₃ | 17.9 ppm CaCO₃ |
| Source | Baking soda | Calcium, magnesium (Equilibrium) |
💡 KH is about stability, GH is about minerals for the fish. Both matter, but KH is more critical.
Target values
| Parameter | Keeping | Breeding | Wild |
|---|---|---|---|
| KH | 2–5 dKH | 1–2 dKH | <1 dKH |
| GH | 3–8 dGH | 2–4 dGH | 1–3 dGH |
⚠️ Don't drop KH under 2 dKH in a keeping tank — a pH crash is guaranteed.
Why KH = anti pH crash
pH is naturally unstable — CO₂ from the fish's respiration lowers pH at night. Without enough KH, pH falls uncontrolled:
| KH | pH swing over 24h | Risk |
|---|---|---|
| 5+ dKH | ±0.1 | ✅ Stable |
| 2–4 dKH | ±0.2–0.3 | ✅ Acceptable |
| 1–2 dKH | ±0.3–0.5 | 🟡 Caution |
| <1 dKH | >0.8 (can fall under 5.0) | 🛑 Guaranteed pH crash |
A pH crash is a sudden drop of more than 1.0 units within hours. Discus can die within 24–48h from the instability — note that ammonia's toxic form (NH₃) shifts with pH, so a crashing, uncontrolled tank is deadly even when individual readings look acceptable.
How to measure KH and GH
Liquid tests (the most accurate):
- JBL Test Combi Set — includes KH+GH
- API GH & KH Test Kit — cheap and reliable
- Sera kH-Test + gH-Test
- Tetra Test Hardness
Method: add drops until the color changes (green → yellow for KH, green → red for GH). The number of drops = the value in dKH/dGH.
Frequency:
- Stable tank: monthly
- After a large RO water change: immediately
- During breeding: weekly
How to raise KH
Baking soda (NaHCO₃) ⭐⭐⭐⭐⭐
The simplest and cheapest. Ordinary supermarket baking soda.
Dose: 0.5 g / 10 L = +1 dKH and +0.2 pH
Calculation: to go from 1 dKH to 3 dKH in a 200 L (~53 US gal) tank:
- Difference: 2 dKH
- Amount: 2 × 0.5 g × 20 (for 200 L) = 20 g baking soda
⚠️ Raise gradually: max +1 dKH/day to avoid shocks. Add it pre-dissolved in water, NOT directly.
Crushed coral in the filter ⭐⭐⭐⭐
Coral (CaCO₃) dissolves gradually as pH drops — a natural buffer.
- Amount: ~100 g / 100 L (~26 US gal)
- Lasts: 6–12 months
- Advantage: self-regulating (you can't overdose)
Seachem Alkaline Buffer ⭐⭐⭐
A commercial product. Dose and instructions on the label. More expensive but precise.
How to raise GH
Seachem Equilibrium ⭐⭐⭐⭐⭐
1 g / 10 L = +3 dGH (no effect on KH).
For GH 5 in a 200 L (~53 US gal) tank:
- 200 L × 1g/10L × (5/3) = ~33 g Equilibrium
Remineralizer salts
- Salty Shrimp Discus Mineral GH+ — for breeding
- Dennerle Osmose Remineral+ — balanced KH+GH
- Tropic Marin Re-Mineral — premium
Magnesium sulfate (Epsom salt)
To raise magnesium only — useful if total GH is fine but magnesium is low. 0.5 g / 10 L = +1 dGH of magnesium.
How to lower KH/GH
The only method: dilution with RO water (reverse osmosis).
Formula: final KH = (current KH × current volume) / total volume after change
Example: a 100 L tank at KH 8 dKH. Remove 50 L, add 50 L RO water (KH 0):
- final KH = (8 × 50) / 100 = 4 dKH ✓
For breeding, gradually shift to 70–80% RO water for KH 1–2 dKH.
Common mistakes
❌ KH 0 after large RO water changes → overnight pH crash ❌ Baking soda added directly to the tank, undissolved → local shock ❌ Using a phosphate-based "pH buffer" → feeds algae ❌ Coral in the filter without watching the target pH → KH climbs over 8 dKH ❌ Testing KH/GH with strips (±2 dKH error, useless) ❌ Confusing KH with GH → you add a remineralizer but the pH crash continues
Knowing your tap water
Tap hardness varies enormously by region. Test your own KH and GH before deciding.
| Tap reading | Decision |
|---|---|
| KH 4–6, GH 8–12 (soft–medium) | ✅ Often fine directly for farm-raised strains |
| KH 6–8, GH 12–16 (medium) | 🟡 Mix ~25–40% RO |
| KH 8–12, GH 14–20 (hard) | Mix 50% RO |
| KH over 12, GH over 20 (very hard) | Mix 60% RO or use RO + remineralization |
KH/GH checklist
- ☐ KH tested monthly — value 2–5 dKH
- ☐ GH tested monthly — value 3–8 dGH
- ☐ KH at least 2 dKH (anti pH crash)
- ☐ Baking soda on hand for emergencies
- ☐ RO water remineralized with KH+GH if you use RO
- ☐ Coral in the filter for gradual buffering (optional)
- ☐ KH tested after every large water change
- ☐ Regular 25%/week changes with target-KH/GH water
What to do next
📚 Learn more:
🧮 Calculate:
🛒 Buy:
- Verified breeders and trusted stores for KH/GH tests (API, JBL, Sera) and remineralizers (Seachem Equilibrium, Alkaline Buffer).
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