TDS values for discus: 100–200 ppm for keeping, 50–100 ppm for breeding
TDS measures the dissolved minerals in your water. For discus: 100–200 ppm for keeping, 50–100 ppm for breeding. How to measure, control it with RO, and remineralize safely.
Quick summary
- Keeping TDS: 100–200 ppm for farm-raised strains.
- Breeding TDS: 50–100 ppm — below that, fertilization fails.
- Wild discus (Heckel, blackwater): under 50 ppm required.
- Measure with a digital TDS meter (e.g. HM Digital TDS-3).
- Pure RO water = 0 TDS = lethal without remineralization.
- Lower it by mixing RO + tap water, using a simple percentage formula.
In short
- Keeping TDS: 100–200 ppm for farm-raised discus.
- Breeding TDS: 50–100 ppm (essential for fertilization).
- Wild discus (Heckel/blackwater): under 50 ppm required.
- How to measure: a digital TDS meter (e.g. HM Digital TDS-3).
- How to lower it: RO (reverse osmosis) water mixed with tap.
- Pure RO water = 0 TDS → lethal without remineralization (Seachem Equilibrium, Salty Shrimp).
What you'll learn
TDS (Total Dissolved Solids) is the total of dissolved minerals in your water, measured in ppm (parts per million). It includes calcium, magnesium, sodium, sulfates, nitrate and more. Wild discus live in the Rio Negro at TDS 10–30 ppm — almost "distilled" water. Farm-raised fish tolerate 200–400 ppm, but for breeding low TDS is essential. This guide shows you how to measure it, lower it with RO, and remineralize correctly.
Target values by scenario
| Scenario | Target TDS | Water source |
|---|---|---|
| Keeping (farm-raised) | 100–250 ppm | Tap + mix |
| Keeping (wild/Asian) | 50–150 ppm | RO + remineralization |
| Breeding | 50–100 ppm | 80% RO + remineralization |
| Fry (first 2 months) | 80–150 ppm | Stable |
| Heavily planted tank | 150–250 ppm | Plant/discus compromise |
Knowing your tap water TDS
Tap water TDS varies enormously by region — even from one street to the next, depending on pipe age. Test your own supply before deciding.
| Tap reading | Decision for discus |
|---|---|
| Under 250 ppm, soft | ✅ Often fine directly |
| 250–350 ppm, medium | 🟡 Mix ~25–35% RO |
| 350–450 ppm, hard | Mix 40–50% RO |
| Over 500 ppm, very hard | Mix 50–60% RO |
How to measure TDS
A digital TDS meter — essential
| Model | Note |
|---|---|
| HM Digital TDS-3 ⭐ | The most popular, reliable |
| HM Digital AP-2 | With a thermometer |
| Hanna HI98301 Dist 1 | Premium, ±2% accuracy |
| Milwaukee TS-01 | Reliable, good warranty |
How to measure:
- Turn the meter on
- Dip the probe 2–3 cm (~1 in) into the water
- Swirl gently for 10 seconds
- Wait for the number to stabilize (~15s)
- Read in ppm
Calibration: once a year with a 342 ppm or 1000 ppm reference solution.
How to lower TDS
RO (reverse osmosis) water ⭐⭐⭐⭐⭐
The only reliable method. RO water has a TDS of 0–10 ppm.
RO options:
| Source | Pros |
|---|---|
| Bought by the bottle | Simple, no investment |
| Home RO system | Low long-term cost per liter |
| RO + DI (deionization) | TDS 0, for breeding |
Recommended RO systems:
- A 5-stage RO unit (e.g. Aquafilter RX-RO5)
- A premium unit (e.g. SpectraPure) for serious breeders
Quick RO + tap mix calculation
Formula: final TDS = (RO TDS × RO% + tap TDS × tap%) / 100
Example: tap TDS = 400 ppm. You want a final TDS of 150 ppm:
- 60% RO (0 ppm) + 40% tap (400 ppm) = 0 + 160 = 160 ppm ✓
🧮 Use the RO + tap water mixer — it calculates the percentages automatically.
Remineralizing pure RO water
100% pure RO (TDS 0) = lethal for discus — no minerals, KH 0, a guaranteed pH crash.
Add minerals back with:
| Product | Dose | Effect |
|---|---|---|
| Seachem Equilibrium ⭐ | 1 g / 10 L | +3 dGH, 0 KH |
| Salty Shrimp Discus Mineral GH+ | 1 g / 10 L | +3 dGH, Amazon minerals |
| Tropic Marin Re-Mineral | 1 g / 8 L | +3 dGH, balanced |
| Dennerle Osmose remineral+ | 1 g / 10 L | +1.5 dGH, +0.5 dKH |
| Baking soda (NaHCO₃) | 0.5 g / 10 L | +1 dKH, +0.2 pH |
Recommended breeding combo (per 100 L / ~26 US gal):
- 80% RO water
- 20% tap water
-
- Seachem Equilibrium (GH 3–5)
-
- baking soda (KH 2)
- Result: TDS ~80 ppm, pH 6.5, KH 2, GH 4 ✓
TDS rises over time — why?
In the tank, TDS slowly climbs through:
- ⬆ Fish and plant waste
- ⬆ Decaying food
- ⬆ Accumulated nitrate
- ⬆ Evaporation (minerals stay, water leaves)
Fix: regular 25–30%/week changes with target-TDS water. Don't "top off" evaporation with tap water — you concentrate the minerals; top off with RO instead.
Common mistakes
❌ 100% RO water with no remineralization → pH crash ❌ Topping off evaporation with tap water → TDS climbs weekly ❌ Using commercial distilled water (not remineralized) ❌ An uncalibrated TDS meter (20–30% error) ❌ Ignoring TDS because pH "seems ok" — TDS shows saturation ❌ Breeding at TDS 300+ → eggs don't fertilize
A long-term RO system
If you have 2+ tanks or plan to breed, a home RO system often pays for itself within 6–12 months versus buying water by the bottle (a Romania estimate; do your own cost comparison locally).
Recommended setup:
- A 5-stage RO unit (e.g. Aquafilter or Aquatec)
- A 50–100 L storage tank
- A permanent inline TDS meter
- An optional DI filter (for absolute TDS 0, breeding)
TDS checklist
- ☐ Digital TDS meter calibrated (once a year)
- ☐ Value 100–250 ppm for keeping
- ☐ Value 50–100 ppm for breeding
- ☐ RO water remineralized (NOT pure)
- ☐ 25%/week changes with target-TDS water
- ☐ Don't top off evaporation with tap water
- ☐ TDS tested weekly or before a change
- ☐ KH at least 2 dKH after remineralization
What to do next
📚 Learn more:
🧮 Calculate:
🛒 Buy:
- Verified breeders and trusted stores for TDS meters and remineralizers (Seachem Equilibrium, Salty Shrimp).
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