The ideal pH for discus: 6.0–6.8 and how to keep it stable
The ideal pH for discus is 6.0–6.8 (5.5–7.5 acceptable for farm-raised strains). How to lower it with peat/RO/catappa, avoid a pH crash, and why stability beats a perfect number.
Quick summary
- Optimal keeping pH: 6.0–6.8 (5.5–7.5 acceptable for farm-raised strains).
- Breeding: 5.5–6.5 with TDS under 100 ppm.
- Wild discus (Heckel and blackwater forms): 5.0–6.0 required.
- Stability beats a perfect value — max swing ±0.3 pH/day.
- Lower pH with RO water, aquarium peat, or catappa leaves.
- Don't use commercial pH down/up — it causes a pH crash.
In short
- Optimal pH: 6.0–6.8 for farm-raised discus (commercial strains).
- Acceptable: 5.5–7.5 — modern discus tolerate harder water.
- Breeding: 5.5–6.5 (TDS under 100 ppm).
- Wild discus (Heckel, blackwater forms): 5.0–6.0 required.
- Stability beats a perfect value — max swing ±0.3 pH/day.
- Don't use commercial pH down/up — it destabilizes the tank and causes a pH crash.
What you'll learn
pH measures water acidity on a 0–14 scale (7 = neutral). Discus prefer acidic-to-neutral water (6.0–7.0) because they come from the Amazon, where the waters are acidified by leaf tannins. This guide gives you the target values, 3 safe ways to lower pH, how to avoid a pH crash (the most dangerous mistake), and why "quick chemicals" are a trap. For the full parameter table, see Ideal water parameters.
Target values by scenario
| Scenario | Target pH | Note |
|---|---|---|
| Daily keeping (farm-raised) | 6.5–7.2 | 7.5 acceptable |
| Breeding | 5.5–6.5 | TDS under 100 ppm |
| Growing fry | 6.5–7.0 | Stable |
| Wild discus (Heckel) | 5.0–6.0 | Required |
| Disease treatment | 6.5–7.0 | Don't change during treatment |
Why stability matters more
A swing over ±0.3 pH/day is more stressful than a stable "wrong" pH. Discus in a tank at a constant pH 7.4 are healthier than those in a tank swinging between 6.5 and 7.2 daily.
Common causes of instability
| Cause | Effect | Fix |
|---|---|---|
| KH below 2 dKH | pH crash overnight | KH at least 2–3 dKH |
| CO₂ injected without a solenoid | pH drops 0.4–0.8 at night | Solenoid on a timer |
| Large water change with a different pH | Shock | Changes of 20–25% max |
| Overstocked tank | CO₂ at night → pH ↓ | Reduce stock or increase aeration |
How to lower pH (from 7.5 to 6.5)
Method 1: RO (reverse osmosis) water ⭐⭐⭐⭐⭐
The safest. Mix tap water with RO water in ratios:
| Target pH | RO % | Tap % |
|---|---|---|
| 7.2 | 30% | 70% |
| 6.8 | 50% | 50% |
| 6.5 | 65% | 35% |
| 6.0 | 80% | 20% |
These ratios are a starting point for moderately hard tap water (around pH 7.5). Always test your own supply.
Method 2: Aquarium peat ⭐⭐⭐⭐
Put granular peat (e.g. JBL Tormec or Fluval Peat) in the filter — it releases humic acids that lower pH gradually by 0.3–0.8 units.
- Amount: roughly 50 g of peat per 100 L (~26 US gal)
- Lasts: 2–4 weeks before recharging
- Bonus: lightly amber-tinted water (natural for discus)
Method 3: Catappa leaves + mopani wood ⭐⭐⭐⭐
Indian Almond leaves (Terminalia catappa) + mopani wood release tannins that:
- Lower pH by 0.2–0.5 units
- Have a natural antibacterial effect
- Reduce fish stress
Amount: 1–2 leaves per 50 L (~13 US gal), replaced every 2–3 weeks.
Method 4 (NOT recommended): chemical pH down ❌
"pH down" products based on phosphoric/sulfuric acid:
- Drop pH abruptly (osmotic shock)
- Need constant reapplication
- Don't fix the cause (water with no buffering capacity)
- Risk a pH crash when you stop
Never in a discus tank.
How to raise pH (from 5.5 to 6.5)
Rarely needed with discus, but if your water is too acidic (pure RO, lots of wood):
- Raise KH with baking soda (NaHCO₃) — 1 g per 100 L raises KH by ~1 dKH and pH by ~0.2
- Partial change with tap water (harder)
- Crushed coral in the filter (releases carbonates gradually)
⚠️ Raise pH by a maximum of 0.3/day.
pH crash — the most dangerous mistake
A pH crash is a sudden drop of more than 1.0 pH within hours. Water with KH under 1 dKH becomes unstable; pH can fall overnight from 6.5 to under 5.0. Discus can die within 24–48h — note that ammonia toxicity changes with pH (it is far less toxic at very low pH, but the instability itself and any subsequent correction are deadly), so never let pH swing freely.
Prevention:
- ☐ KH always at least 2 dKH
- ☐ Test KH weekly
- ☐ Regular water changes (25%/week) — they restore KH
- ☐ Be careful with 100% RO water and no remineralization
See KH and GH — water hardness.
Knowing your tap water
Tap water pH varies enormously by region, so test your own before deciding.
| Tap reading | Decision |
|---|---|
| pH 7.0–7.4, soft | ✅ Often fine directly for farm-raised strains |
| pH 7.2–7.6, medium | 🟡 Mix ~40% RO |
| pH 7.6–8.0, hard | 🟡 Mix 50–60% RO, or use peat |
| pH over 8.0 / limestone | RO strongly recommended |
Common mistakes
❌ Using chemical "pH down/up" ❌ Changing pH during a medication treatment ❌ A 50%+ water change with a different pH (shock) ❌ A tank with KH 0 (pure RO water) and no remineralization ❌ Reading pH right after feeding (respiratory CO₂ skews it) ❌ Testing pH with strips (±0.5 pH error, useless)
pH checklist
- ☐ pH measured weekly with a liquid test (NOT strips)
- ☐ Value between 6.0 and 7.5
- ☐ Stability ±0.3 pH/day
- ☐ KH ≥ 2 dKH (anti pH crash)
- ☐ Regular changes 25% × 2/week
- ☐ Change water matched on temperature and pH (±0.3)
- ☐ No "pH down/up" products in the tank
- ☐ pH tested 2 hours after a water change
What to do next
📚 Learn more:
- Ideal water parameters — the complete table
- KH and GH — anti pH crash
- How to test the water correctly
🧮 Calculate:
🛒 Buy:
- Verified breeders and trusted stores for liquid pH tests (API, JBL, Sera), peat and conditioners.
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