Mixing RO + tap water for discus: ratios and TDS calculation
Blending RO with tap water is the most practical way to give discus stable, soft-enough water. Here's the simple formula, a ratio table by tap hardness, and how to remineralize when you use mostly RO.
Quick summary
- Formula: final TDS ≈ tap TDS × (tap fraction). RO is near 0 TDS.
- Soft tap (under ~250 ppm): often no mix needed for farm-raised discus.
- Medium tap (~300–400 ppm): blend ~50% RO to land around 150–200 ppm.
- Hard tap (over ~400 ppm): 55–65% RO to reach a discus-friendly TDS.
- Breeding: mostly RO (~80%) + a precise remineralizer (low TDS).
- 100% pure RO is LETHAL without remineralization — never use it neat.
In short
- Why blend: many tap supplies run TDS 250–600 ppm; discus prefer roughly 100–200 ppm.
- The formula: final TDS ≈ tap TDS × tap fraction (RO is near 0 TDS, so it just dilutes).
- Soft tap (under ~250 ppm): farm-raised discus are often fine with dechlorinated tap, no RO needed.
- Medium tap (~300–400 ppm): ~50% RO lands you around 150–200 ppm.
- Hard tap (over ~400 ppm): 55–65% RO to reach a discus-friendly target.
- Pure RO (TDS 0): mandatory remineralization — neat RO causes a guaranteed pH crash.
What you'll learn
Reverse-osmosis (RO) water has almost no dissolved minerals (TDS near 0), so it's ideal for diluting mineral-heavy tap water down to a discus-friendly level — without stripping out everything the way 100% RO would. This guide gives you the mixing formula, a ratio table organized by how hard your tap water is, and how to safely handle mostly-RO blends for breeding. For background on the target itself, see TDS values for discus.
Start by knowing your tap water
Tap hardness varies enormously between regions, cities and even neighborhoods — so the single most important step is to measure your own tap water rather than copy someone else's ratio.
| Your tap TDS | Typical character | RO fraction for ~150–200 ppm |
|---|---|---|
| Under 250 ppm | Soft–medium | 0–20% RO (often none needed) |
| 250–350 ppm | Medium | 20–40% RO |
| 350–450 ppm | Hard | 40–55% RO |
| 450–600 ppm | Very hard | 55–65% RO |
| Over 600 ppm (e.g. well water) | Extremely hard | 65–80% RO + retest |
💡 Buy an inexpensive handheld TDS meter and measure your tap — readings drift between neighborhoods and seasons.
The mixing formula
Final TDS
final TDS = (tap TDS × tap fraction) + (RO TDS × RO fraction)
Because RO TDS ≈ 0 ppm, it simplifies to:
final TDS ≈ tap TDS × tap fraction
To find the ratio for a target TDS
tap fraction = target TDS ÷ tap TDS, then RO fraction = 1 − tap fraction.
Worked examples
Example 1 — hard tap (400 ppm), target 180 ppm:
- tap fraction = 180 ÷ 400 = 0.45 → 45% tap
- 55% RO
Example 2 — medium tap (240 ppm), target 180 ppm:
- tap fraction = 180 ÷ 240 = 0.75 → 75% tap
- 25% RO
Example 3 — breeding target (80 ppm) from hard tap (400 ppm):
- tap fraction = 80 ÷ 400 = 0.20 → 20% tap
- 80% RO + KH remineralization
🧮 The RO + tap water mixer does this automatically from your own readings.
Volumes for a 100 L (≈26 US gal) batch
Once you know the fractions, the volumes are easy:
| RO fraction | RO water | Tap water |
|---|---|---|
| 25% | 25 L (≈6.6 gal) | 75 L (≈19.8 gal) |
| 50% | 50 L (≈13.2 gal) | 50 L (≈13.2 gal) |
| 55% | 55 L (≈14.5 gal) | 45 L (≈11.9 gal) |
| 80% | 80 L (≈21.1 gal) | 20 L (≈5.3 gal) |
Remineralizing mostly-RO blends
When your blend is more than ~70% RO, you've removed so much mineral content that you should add some back for KH (pH stability) and GH — otherwise pH becomes unstable.
Keeping blend
| Add | Approx. dose | Effect |
|---|---|---|
| Seachem Equilibrium (or similar GH+) | 1 g / 10 L | +~3 dGH (Ca, Mg, K) |
| Baking soda (NaHCO₃) | 0.5 g / 10 L | +~1 dKH, raises pH slightly |
| Catappa (Indian almond) leaves | 1 leaf / 50 L | tannins, trace minerals |
Typical result: TDS ~80 ppm, KH ~2 dKH, GH ~3 dGH — a soft, stable keeping target.
Breeding blend (TDS under ~100 ppm)
| Add | Approx. dose | Effect |
|---|---|---|
| Salty Shrimp Discus Mineral GH+ | ~0.5 g / 10 L | TDS ~70, GH ~3, balanced minerals |
| Baking soda (NaHCO₃) | ~0.3 g / 10 L | KH ~1.5 dKH |
| Catappa leaves | 2 leaves / 50 L | soft, tannin-rich blackwater |
⚠️ Doses are starting points — always confirm with a TDS meter and a KH test, because product strengths and your RO purity vary.
Mixing — step by step
- Set up two containers — one tap, one RO
- Measure the TDS of each with a meter
- Calculate the volumes (formula above)
- Pour the RO water into the larger container first
- Add the remineralizer (if you're using more than ~70% RO)
- Add the tap water gradually
- Stir well for ~30 seconds
- Check the final TDS with the meter
- Add a dechlorinator (e.g. Seachem Prime, 1 ml / 40 L)
- Heat to tank temperature (around 29°C / 84°F) before the change
Buying RO vs. owning a system
You can either buy RO water (from aquarium shops or some retailers) or run your own RO unit at home. The trade-off:
| Option | Upfront | Convenience | Best for |
|---|---|---|---|
| Buying RO water | None | Low (you haul it) | One small tank, occasional use |
| Home RO unit | Higher | High (on tap) | 2+ tanks, large tank, breeding |
💶 Exact prices vary widely by country and brand, so compare locally. In general, a home unit pays off once you're moving a lot of water each month or running multiple tanks. (Any specific figures you see elsewhere on this site are Romania estimates and won't match your region.)
Common mistakes
❌ Pure RO (100%) straight into the tank → guaranteed pH crash ❌ Guessing the ratio without a TDS meter → big swings between batches ❌ Topping off evaporation with tap water → TDS creeps up week by week (use RO for top-offs) ❌ Blending well water with RO without testing → well water can be 600–1500 ppm and may carry iron/nitrate ❌ Storing RO in metal containers → contamination (use food-grade plastic) ❌ Mixing directly in the tank instead of a separate container → local swings, stress
A note on well water
Well water can run TDS 600–1500 ppm — often harder than municipal tap — and may carry iron, manganese or agricultural nitrate. If you're on a well: test TDS, pH, KH, GH, nitrate and iron before using it, plan for a higher RO fraction (70–80%), and add an iron-removal stage if iron is high.
Mixing checklist
- ☐ TDS meter on hand and calibrated
- ☐ RO source working (RO output reads under ~10 ppm)
- ☐ Tap water tested (TDS, pH, KH)
- ☐ Ratio calculated for your target TDS
- ☐ Remineralizer ready if RO is over ~70% (Equilibrium + baking soda)
- ☐ Dechlorinator added (e.g. Seachem Prime)
- ☐ Water mixed in a separate container (not in the tank)
- ☐ Temperature matched to the tank before the change
- ☐ Final TDS and pH checked after mixing
What to do next
📚 Learn more:
- TDS values for discus
- KH and GH — water hardness
- The ideal pH for discus
- How often to change the water
🧮 Calculate:
- RO + tap water mixer — automatic ratios
- Aquarium unit converter — TDS ppm ↔ µS/cm
🛒 Buy:
- Verified breeders and trusted stores for TDS meters, water tests, remineralizers and RO systems.
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