Discus Fish Care
Complete UK Guide
Discus are the most demanding freshwater fish kept in UK aquariums — but also the most rewarding. This guide covers everything specific to UK keepers: water treatment, RO ratios for your region, disease identification and full husbandry protocol.
By Hasan, Discus Specialist & Aquascaping Lead, AD Aquatics · Updated 2025
Why Discus Are Different
Discus (Symphysodon spp.) are native to the Amazon basin — specifically blackwater tributaries with near-distilled water chemistry. In the wild, pH can drop to 4.5, GH is often below 1°dH, and temperatures reach 30–32°C. UK tap water is the opposite: hard, alkaline, and cool. This gap is why discus are classified as "expert" fish and why most UK hobbyists need reverse osmosis equipment before attempting them.
Hasan at AD Aquatics has kept and maintained discus systems for over 4 years. The information below reflects what actually works in UK conditions — not theoretical parameters from tropical literature.
Water Parameters
| Parameter | Acceptable Range | Optimal Range | Critical Threshold | Notes |
|---|---|---|---|---|
| pH | 5.5–7.0 | 6.0–6.8 | >7.2 causes chronic stress | Stable pH more important than exact value — avoid swings >0.2 in 24hrs |
| Temperature | 27–32°C | 28–30°C | <26°C immune suppression | Higher than any common tankmate — limits compatible species significantly |
| GH (General Hardness) | 1–10°dH | 2–6°dH | >12°dH spawning failure | UK tap water typically 10–25°dH — RO essential in most regions |
| KH (Carbonate Hardness) | 0–5°dH | 1–3°dH | >6°dH blocks pH reduction | KH buffers pH — high KH prevents achieving target pH even with acidifiers |
| Ammonia | 0 ppm | 0 ppm | >0.1 ppm disease trigger | Discus are extremely sensitive — any detectable ammonia is dangerous |
| Nitrite | 0 ppm | 0 ppm | >0.1 ppm | As above — never add discus to uncycled or recovering tank |
| Nitrate | 0–20 ppm | <10 ppm | >30 ppm disease trigger | Requires 3× weekly water changes in most setups |
RO Water: UK Regional Mixing Ratios
Reverse osmosis removes virtually all minerals, producing water at GH 0–1°dH. You then blend RO water with tap water to reach your target, then reconstitute with trace minerals. Target GH for discus: 3–5°dH.
| UK Region | Tap GH (°dH) | % RO Water | % Tap Water | Final GH | Reconstitute With |
|---|---|---|---|---|---|
| Thames Valley / SE England | 20–25 | 85–90% | 10–15% | ~3–4°dH | Seachem Discus Buffer + Equilibrium (micro-dose) |
| East Anglia | 18–22 | 80–85% | 15–20% | ~3–4°dH | As above |
| Midlands | 14–18 | 75–80% | 20–25% | ~3–4°dH | As above |
| Yorkshire / NE England | 10–16 | 70–78% | 22–30% | ~3–4°dH | Seachem Discus Buffer only |
| NW England (Manchester/Liverpool) | 6–10 | 55–65% | 35–45% | ~3–4°dH | Seachem Discus Buffer |
| Wales | 2–8 | 30–50% | 50–70% | ~3°dH | May need trace minerals only |
| Scotland (most regions) | 2–6 | 20–40% | 60–80% | ~3°dH | pH adjustment only (Seachem Discus Buffer) |
| SW England / Cornwall | 4–10 | 40–60% | 40–60% | ~3°dH | Seachem Discus Buffer |
Values are approximate — always test your actual tap water before calculating ratios. Use Seachem Discus Buffer to lower pH in the mixed water. Test final pH and GH before adding to tank. See our UK tap water hardness guide for precise regional data.
Feeding Schedule
Discus are mid-water feeders that eat frequently in the wild. In captivity:
- Adults: 3–4 small meals per day (feed what is eaten in 2–3 minutes)
- Juveniles (under 3 months): 5–6 meals daily — growth rate depends heavily on feeding frequency
- Primary diet: High-quality discus granules/pellets as the base. JBL NovoDisco, Tetra Discus, and Hikari Discus Bio-Gold are well-regarded UK products
- Protein supplementation: Frozen bloodworm, frozen white mosquito larvae, and beef heart (prepared) 2–3× per week. Vary protein sources to prevent nutritional deficiencies
- Avoid: Feeder fish, raw beef heart with excessive fat (causes fatty liver), dried foods only (insufficient moisture)
- Fasting: One fast day per week helps gut health and reduces nitrate loading
Tankmates for Discus
The 28–32°C temperature eliminates most community fish. Compatible species:
- Cardinal tetras (Paracheirodon axelrodi) — the classic discus companion; tolerates 28–30°C and soft acidic water
- Sterbai corydoras (Corydoras sterbai) — the only corydoras species comfortable at 28–30°C; provides valuable substrate cleaning
- Altum angelfish (Pterophyllum altum) — shares blackwater origins; however can compete aggressively at feeding time
- Rummy nose tetras (Hemigrammus rhodostomus) — shoaling fish that tolerates discus temperatures
- Avoid: Most corydoras (too cool), neon tetras (temperature intolerant), any aggressive or nippy species
Frequently Asked Questions
Do discus need RO water in the UK?
Yes, in virtually all UK regions. Most UK tap water is GH 10–25°dH — well above discus tolerances of 1–8°dH. Scotland, Wales, and parts of northwest England have softer water, but still typically require 20–50% RO blending. See the RO mixing ratio table above for your region.
How often should I change water in a discus tank?
30–50% every 2–3 days is the standard for a healthy discus display tank. This keeps nitrate below 20 ppm. Daily changes are used by breeders but are not necessary for a well-filtered non-breeding display tank. Never skip more than 4 days between water changes.
How many discus in a 200-litre tank?
6 discus minimum — they are schooling fish and develop stress-related diseases when kept in pairs or trios. A 200-litre tank (120 × 40 cm footprint, 50 cm height minimum) suits 6 discus comfortably. A 300-litre tank allows 8–10. Never keep a lone discus.
Why are my discus turning dark and hiding?
Darkening is a primary stress indicator in discus. Common causes: water quality issue (test immediately — especially pH and nitrate), temperature drop below 27°C, new fish introduction causing hierarchy stress, illness (check for white stringy faeces indicating internal parasites), or inadequate group size. Hasan at AD Aquatics can diagnose remotely with water test results and photos — contact us.
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