Acclimating Asian discus to hard water: a 6-week transition
Acclimating discus from Asia to hard tap water — a slow 6-week schedule with TDS, pH and GH steps, the warning signs of osmotic shock, plus the equipment for a correct transition.
Quick summary
- Asian discus are raised at TDS 100–150 ppm, pH 5.5–6.5, GH 3–5 — SOFT blackwater-biotope water.
- Hard tap water is often TDS 250–450 ppm, pH 7.0–7.8, GH 8–15 — HARD water.
- An ABRUPT transition = lethal osmotic shock in 2–7 days.
- Correct transition: 6 weeks, raising TDS +25 ppm/week and pH +0.2 units/week at most.
- Required equipment: an RO unit, a remineralizer, a digital TDS meter, a pH meter, a GH/KH test kit.
- Osmotic-shock signals: permanent stress bars, food refusal, clamped fins, white feces, fast breathing.
In short
- Asian discus are raised at TDS 100–150 ppm, pH 5.5–6.5, GH 3–5 — SOFT, blackwater-biotope water.
- Hard tap water is often TDS 250–450 ppm, pH 7.0–7.8, GH 8–15 — HARD water (common in many European cities).
- An ABRUPT transition = lethal osmotic shock within 2–7 days.
- Correct transition: 6 weeks, raising TDS by +25 ppm/week and pH by +0.2 units/week at most.
- Required equipment: an RO unit, a remineralizer, a digital TDS meter, a pH meter, a GH/KH test kit.
- Osmotic-shock signals: permanent stress bars, food refusal, clamped fins, white feces, fast breathing.
💱 Equipment prices vary by country. Where figures appear, they're labelled as a Romania estimate (2026) — use your local market. The schedule itself is universal: it depends on your water's TDS/pH, not your country.
What you'll learn
Acclimating Asian discus to hard water is a 6-week protocol for gradually transitioning a discus raised in Asia (soft water, TDS 100–150 ppm) to your water's parameters (often TDS 250–450 ppm from a hard tap supply). An ABRUPT transition causes osmotic shock — the fish's body can't suddenly balance the osmotic pressure, leading to cellular edema, food refusal, chronic weakness and high mortality within 2–14 days. This guide gives you the exact weekly schedule, the equipment needed, the signs that the transition is too fast and how to correct course if problems appear.
Why a slow transition is critical
The osmotic difference: Asia vs hard tap water
| Parameter | Asian water (hatchery) | Typical hard tap | Difference |
|---|---|---|---|
| TDS | 100–150 ppm | 250–450 ppm | +150–300 ppm |
| pH | 5.5–6.5 | 7.0–7.8 | +1–2 units |
| GH | 3–5 dGH | 8–15 dGH | +5–10 dGH |
| KH | 0–2 dKH | 4–10 dKH | +3–8 dKH |
| Temperature | 28–30 °C (82–86 °F) | 28–30 °C (82–86 °F) | Identical ✅ |
A brutal transition = the fish's gill osmoregulation absorbs too much salt too fast → cellular edema → death in 2–7 days.
What osmotic shock does at the cellular level
- Gills — the gill epithelium cells swell and block gas exchange → fast breathing, surface gasping
- Kidneys — unable to excrete the surplus salts → a buildup of Na⁺ and Cl⁻ ions in the blood
- Nervous system — electrolyte imbalances → disoriented, tilted swimming
- Slime coat — excess production for protection → "haze" on the body
- Immune system — collapses → latent pathogens activate (Hexamita, flukes)
RESULT: high mortality within 7–14 days if you don't intervene.
The equipment you need
| Equipment | Function |
|---|---|
| RO unit (50–100 GPD) | The base water for the transition |
| Digital TDS meter | Daily measurement |
| Digital pH meter (or a liquid test) | Check a stable pH |
| GH/KH test kit | Monitor hardness |
| A remineralizer (e.g. Tropic Marin Re-Mineral, Seachem Equilibrium) | Controlled TDS increase |
| A 25–50 L (6.6–13 US gal) container | Pre-mixing transition water |
| A pH-lowering buffer (acid buffer) | Lower the pH |
| A pH-raising buffer (alkaline buffer) | Raise the pH |
🇷🇴 RO estimate (2026): RO unit ~€80–140; TDS meter ~€10–20; pH meter ~€30–50; GH/KH kit ~€16–30; remineralizer ~€12–24/kg. One-time setup ≈ €175–310 + ~€10–20/month consumables. Use your local market.
The weekly schedule — 6 full weeks
Week 0 — Preparation BEFORE the fish arrive
7–14 days before:
- Test your water — measure TDS, pH, GH, KH of both the tap and the RO-filtered water
- Buy the equipment from the list above
- Prepare a 120 L (~30 US gal) bare-bottom quarantine tank with water CLOSE to Asia's (see below)
- Pre-mix 50–100 L (13–26 US gal) of remineralized RO water at the initial target
Target parameters, quarantine tank, Day 1:
| Parameter | Value | How to reach it |
|---|---|---|
| TDS | 130–150 ppm | RO + a very little remineralizer |
| pH | 6.5–6.8 | natural RO (pH ~6.5) |
| GH | 4–5 dGH | dosed remineralizer |
| KH | 2–3 dKH | a very little buffer |
| Temp | 30 °C (86 °F) | heater |
Why not the EXACT Asian water (TDS 100, pH 5.5)? Because the risk of pH instability at 5.5 (below minimum KH) outweighs the benefit. We start at 130–150 ppm = ~80% similar to Asia, but STABLE.
Week 1 — Acclimating to the new environment (NO parameter changes)
Goal: the discus gets used to the new tank, food and lights. You don't change the water parameters.
| Activity | Detail |
|---|---|
| Water changes | 20% daily with pre-mixed water at THE SAME parameters (TDS 130–150, pH 6.5–6.8) |
| Feeding | 1–2×/day, premium pellets, NO beef heart |
| Observation | Stress bars, feces, breathing, appetite |
| Treatments | NO treatment in week 1 |
| Lights | Dimmed, 4–6 hours/day |
Normal signals, week 1: visible stress bars the first 2–3 days, weak appetite the first 24–48h, pale colors. These normalize after day 4–5.
Week 2 — Preventive treatment (parameters still constant)
Apply a dual treatment of an antiparasitic + an anti-protozoal (e.g. praziquantel + metronidazole) with a 7-day gap between them. See the quarantine protocol.
IMPORTANT: keep the water parameters CONSTANT in week 2 (TDS 130–150, pH 6.5–6.8). Don't start the transition during treatments — combined stress = higher mortality.
Week 3 — START the slow transition
Goal: gradually raise TDS, pH and GH in SMALL steps.
Steps per day (week 3)
| Day | Target TDS | Target pH | How |
|---|---|---|---|
| Mon | 150 | 6.8 | +5 ppm via a 20% change with remineralized water |
| Wed | 155 | 6.85 | +5 ppm via a 20% change |
| Fri | 165 | 6.9 | +10 ppm + a little buffer |
| Sun | 175 | 7.0 | +10 ppm |
| End of week 3 | 175 ppm | 7.0 | +45 ppm vs start |
RULE: at most +25 ppm TDS / week and at most +0.2 pH / week. Faster = osmotic shock.
Week 4 — Continue the transition
| End of week | Target TDS | Target pH | GH |
|---|---|---|---|
| End of week 4 | 200 ppm | 7.1 | 6–7 dGH |
Change 20–25% every 2 days with water +5 ppm above the current tank.
Week 5 — Approaching the main water
| End of week | Target TDS | Target pH | GH |
|---|---|---|---|
| End of week 5 | 225 ppm | 7.2 | 8 dGH |
Test the MAIN tank's parameters — it should be ~250 ppm TDS, pH 7.3–7.4. Aim to close the gap.
Week 6 — End of transition + reintroduction
| End of week | Target TDS | Target pH | GH |
|---|---|---|---|
| End of week 6 | 245–260 ppm | 7.3 | 9–10 dGH |
Reintroduce to the main tank when the parameters are identical within ±5 ppm TDS and ±0.1 pH:
- Turn off the main tank's lights
- Drip acclimation for 30–45 min (even if the parameters are similar)
- Introduce in the evening
- Watch for aggression for 48h
Signs of osmotic shock — INTERVENE IMMEDIATELY
If you see any of the signals below, STOP the transition immediately and step 1 stage back:
| Signal | Severity | Action |
|---|---|---|
| Permanently intense stress bars | ⚠️ Medium | Pause the transition for 7 days |
| Food refusal 3+ days | ⚠️ Medium | Pause + vitamins + drop TDS 10% |
| Permanently clamped fins | 🚨 Major | Drop TDS 20% immediately |
| Breathing >120/min | 🚨 Major | Drop TDS + increase aeration |
| White stringy feces | 🚨 Major | Possibly active Hexamita — metronidazole |
| Sitting tilted on the bottom | 🚨 Critical | Drop TDS 30% + temperature 30.5 °C (87 °F) |
| "Haze" on the body (excess slime) | 🚨 Critical | Drop TDS 30% (use salt with caution in discus) |
| Disoriented swimming | 🚨 Terminal | Drop TDS 50% + continuous observation |
The recovery rule: when a severe signal appears, go back 50 ppm TDS and hold for 7–10 days before resuming the transition (with smaller steps).
Schedules for special cases
Very hard supply (TDS 320–400, pH 7.5–7.8, GH 12–14)
An extended transition of 7–8 weeks — the gap vs Asia is larger. Steps +25 ppm/week maximum. (Example: many large-city mains.)
Moderately hard supply (TDS 250–300, pH 7.3–7.5, GH 10–12)
A standard 6-week transition, normal steps.
Mildly hard supply (TDS 200–280, pH 7.0–7.4, GH 7–10)
A 5-week transition is possible — the gap vs Asia is smaller.
You use RO + remineralizer in the main tank (TDS 180–220)
Excellent! A transition of just 3–4 weeks — the main water is already moderate.
Practical remineralization equivalents
Raising TDS with a typical remineralizer
| Dose | TDS added |
|---|---|
| 1 g / 10 L (2.6 US gal) RO water | ~50 ppm |
| 0.5 g / 10 L | ~25 ppm |
| 0.2 g / 10 L | ~10 ppm |
Practical example: a 25% change in a 120 L (~30 US gal) tank (30 L / 8 US gal of new water). For +5 ppm TDS in the tank → add ~1.5 g of remineralizer to the new RO water (~25 ppm in the new water × 25% change = +6 ppm in the tank). Exact ppm-per-gram varies by product — calibrate with your TDS meter.
Raising pH with an alkaline buffer
| Dose | pH raised |
|---|---|
| 1 g / 80 L (21 US gal) water | ~+0.05 pH |
| 2.5 g / 80 L | ~+0.1 pH |
Figures are indicative — buffering depends on your KH; verify with your own measurements.
Common acclimation mistakes
❌ "I'll put them straight in the main tank, my water's good anyway" — "good" water for you = total shock for Asian discus. High mortality.
❌ "A 1-hour drip acclimation is enough" — dripping acclimates temperature and acute stress, NOT the 150–300 ppm TDS gap. That takes weeks.
❌ "I'll raise TDS by 50 ppm/week to finish faster" — over +25 ppm/week = osmotic shock + latent pathogens activating.
❌ "I'll use tap water directly without RO" — hard tap water can be TDS 250–400+ ppm. Starting the transition at your value = impossible.
❌ "I won't test TDS, I can just sense when it's right" — a TDS meter is a mandatory investment. Without measurement = a chaotic transition.
❌ "I'll add 1–2 g/L salt to stimulate osmosis" — discus don't tolerate salt like other species. 1 g/L for a long time = stress + weakening.
❌ "I'll do the transition and a praziquantel treatment at the same time" — combined stress = much higher mortality. Treatments first (week 2), then the transition (week 3+).
❌ "Big 50%+ water changes to speed things up" — big changes swing the parameters abruptly = shock. Max 25% per change during the transition.
Hard-water transition checklist
- ☐ A working RO unit
- ☐ A TDS meter, a pH meter, a GH/KH test kit
- ☐ A remineralizer in stock
- ☐ A 25–50 L (6.6–13 US gal) pre-mix container
- ☐ A quarantine tank prepared with SOFT water initially (TDS 130–150)
- ☐ A daily log — TDS, pH, observations
- ☐ The weekly schedule printed
- ☐ Weeks 1–2: CONSTANT parameters + treatments
- ☐ Weeks 3–6: transition +25 ppm TDS / week maximum
- ☐ A rollback plan if osmotic-shock signs appear
- ☐ The main water tested and confirmed (final target parameters)
- ☐ Reintroduction only when within ±5 ppm + ±0.1 pH
What to do next
📚 Full quarantine protocol: 4–8 week quarantine.
🏪 Where to buy: Stendker vs Asian vs local.
🌍 Importing directly: Importing discus from Asia.
💧 Reverse osmosis (RO): Reverse osmosis for discus.
📊 TDS for discus: Optimal discus TDS values.