How many discus in an aquarium: a table by volume + calculator
How many discus fit in an aquarium — the 1 discus per 40–50L (~10–13 US gal) rule, a complete table for 60L, 100L, 200L, 300L, 400L, 500L+. Understocking rules, minimum dimensions and common mistakes.
Quick summary
- Rule of thumb: 1 adult discus per 40–50L (~10–13 US gal) of actual water.
- Under 200L (53 US gal): NOT recommended for discus.
- 200L = 4–5 discus | 300L = 6–8 | 400L = 8–10 | 500L+ = 10–12.
- At least 6 discus per group (below 5, bullying appears).
- Minimum tank height ~45 cm (18 in) — discus are tall fish.
- Understocking > overstocking. Better 5 comfortable than 8 stressed.
In short
- Rule of thumb: 1 adult discus per 40–50L (~10–13 US gal) of actual water.
- Under 200L (53 US gal): NOT recommended — the absolute floor.
- 200L = 4–5 discus | 300L = 6–8 | 400L = 8–10 | 500L+ = 10–12.
- At least 6 discus per group (below 5, bullying appears).
- Minimum tank height ~45 cm (18 in) — discus are tall fish (12–18 cm / 4.7–7 in).
- Understocking > overstocking — 5 comfortable discus beat 8 stressed ones.
What you'll learn
One of the first questions a new keeper asks is "how many discus fit in my tank?" The answer depends on the actual water volume, the tank's dimensions (height, length), whether it's discus-only or a community tank, and on filtration. This guide gives you the full table by volume, the exact rule of thumb and the critical rules that 90% of beginners ignore.
The basic rule
1 adult discus (12–15 cm / 4.7–6 in) per 40–50L (~10–13 US gal) of actual water.
- 40L (~10 US gal) per discus = the maximum acceptable density (with good filtration + 2 water changes/week)
- 50L (~13 US gal) per discus = recommended for beginners and densely planted tanks
- 60L+ (~16 US gal) per discus = a luxury, ideal for breeding
⚠️ Actual water = tank volume − substrate − hardscape − wood. A "250L" (66 US gal) gross tank holds roughly 210–220L (55–58 US gal) of actual water.
The full table by volume
| Aquarium volume (actual) | Dimensions (L×W×H) | Max discus | Ideal discus |
|---|---|---|---|
| 60L (16 US gal) | 60×30×35 cm (24×12×14 in) | ❌ 0 | ❌ 0 |
| 100L (26 US gal) | 80×35×40 cm (32×14×16 in) | ❌ 0 | ❌ 0 |
| 150L (40 US gal) | 80×40×45 cm (32×16×18 in) | ⚠️ 3 | ❌ Not recommended |
| 200L (53 US gal) | 100×40×50 cm (40×16×20 in) | 5 | 4 |
| 250L (66 US gal) | 100×50×50 cm (40×20×20 in) | 6 | 5–6 |
| 300L (79 US gal) | 120×50×50 cm (47×20×20 in) | 7 | 6 |
| 350L (92 US gal) | 120×50×60 cm (47×20×24 in) | 8 | 7 |
| 400L (106 US gal) | 150×60×50 cm (59×24×20 in) | 10 | 8 |
| 500L (132 US gal) | 150×60×60 cm (59×24×24 in) | 12 | 10 |
| 600L+ (159 US gal) | 180×60×60 cm (71×24×24 in) | 15 | 12 |
Why under 200L doesn't work
Discus are large fish (12–18 cm / 4.7–7 in as adults) that:
- Need room for a natural hierarchy — 6 fish in a small tank = bullying.
- Produce a lot of organic waste — a small tank = fast-rising nitrates.
- Require large water changes — 50% × 2/week in a small tank = osmotic stress.
- Need a minimum height of ~45 cm (18 in) — they're tall fish; 35 cm (14 in) tanks force their body horizontal and stress them.
Exception: a 100L (26 US gal) tank for a proven breeding pair (max 2 discus). See Discus breeding tank setup.
Critical rules beyond the math
1. At least 6 discus per group ⚠️
Regardless of volume, at least 6 discus for a healthy hierarchy:
- 4 discus = guaranteed bullying of 1 victim
- 5 discus = frequent bullying of 1–2 victims
- 6+ = a naturally distributed hierarchy
Implication: if you have a 150L (40 US gal) tank (max 3 discus by the rule), it's better not to keep discus at all than to keep 3.
2. Dimensions matter as much as volume
A "tall and narrow" 200L (53 US gal) tank (50×40×100 cm) is NOT good for discus — they prefer long and wide (100×40×50 cm).
Minimum height: ~45 cm (18 in). Discus swim vertically and need room on the vertical axis.
Minimum length: ~80 cm (32 in). To allow a hierarchy with separate zones.
3. Community tank — reduce by 20–30%
If you add tank mates (cardinal tetra, Corydoras, plecos), the bioload rises. Apply:
| Setup | Discus |
|---|---|
| Discus-only 300L (79 US gal) | 7 |
| Community 300L (15 cardinals + 6 Corydoras + 1 Ancistrus) | 6 (one fewer) |
4. Filtration sets the practical limit
| Filtration | Max density |
|---|---|
| 1 external filter, 4× volume/h | 1 discus / 50L (13 US gal) |
| 1 external filter, 6× volume/h | 1 discus / 40L (10 US gal) |
| Sump + external filter (6×+) | 1 discus / 35L (9 US gal) (advanced) |
5. The real test — nitrates 7 days after a water change
Whatever the calculation says, the final verdict is the nitrate test:
- <10 ppm 7 days after a 50% change = understocked or perfect ✅
- 10–20 ppm = at the upper limit 🟡
- >20 ppm = overstocked ❌
Concrete examples
🐠 A 250L (66 US gal) discus-only tank
Math: 250L ÷ 50L/discus = 5 ideal discus, max 6.
Setup: 6 discus of different varieties (Red Turquoise, Blue Diamond, Pigeon Blood, Snake Skin, Leopard, Heckel) = a spectacularly colorful group.
🐠 A 300L (79 US gal) community tank
Math: 300L ÷ 50L/discus − 20% for tank mates = 5–6 discus.
Setup: 6 discus + 15 cardinal tetra + 10 rummynose + 6 Corydoras sterbai + 1 Ancistrus = 38 fish total. See Community tank stocking.
🐠 A 200L (53 US gal) beginner tank
Math: 200L ÷ 50L = 4 ideal discus. But the minimum for a hierarchy is 6 → a conflict.
Options:
- A. Compromise: 4 discus + accept a low bullying risk (a roomy tank, a rare single victim) → possible but not ideal
- B. Recommended: save up for 2–3 months, get a 250L+ (66+ US gal) tank and do it right
🐠 A 400L (106 US gal) luxury tank
Math: 400L ÷ 40L = 10 discus max, 8 ideal.
Luxury setup: 8 discus + 25 cardinal tetra + 12 rummynose + 8 Corydoras sterbai + 2 Ancistrus + 1 L134. Needs filtration of at least 2,400 L/h (~635 US gal/h).
Common mistakes
❌ "I have a 100L (26 US gal) tank, I'll start with 2 discus" — Too small a tank + too small a group = guaranteed bullying + unstable parameters.
❌ "100×40×60 = 240L so I'll keep 5 discus" — A tall, narrow tank: 5 cm (2 in) of substrate + hardscape = ~200L (53 US gal) actual water, and a 100 cm (40 in) length is marginal.
❌ "I'll put 8 discus in 250L (66 US gal) to save space" — Overstocking, high nitrates, aggression, frequent disease.
❌ "The discus are small now, I'll put 10 in 200L (53 US gal)" — They grow to 12–18 cm (4.7–7 in) in 12–18 months. Calculate on the adult size.
❌ "The shop sold me 6 discus for a 120L (32 US gal) tank" — The shop is selling. You carry the responsibility for their health.
A quick mental calculator
Steps:
- Actual volume = gross volume × 0.85 (subtract substrate + hardscape)
- Discus = actual volume ÷ 50 (rounded down)
- If < 6 → don't keep discus or get a bigger tank
- For a community tank → subtract 1 discus from the result
Example: a 300L (79 US gal) gross tank → 300 × 0.85 = 255L (67 US gal) actual → 255 ÷ 50 = 5.1 → 5 ideal discus, max 6.
🧮 For an exact calculation for your tank (substrate, hardscape, tank mates), use the stocking calculator.
Checklist before you buy discus
- ☐ Tank at least 200L (53 US gal), 250L+ (66+ US gal) recommended
- ☐ Tank height at least ~45 cm (18 in)
- ☐ Length at least ~80 cm (32 in)
- ☐ Filtration with a flow of 4–6× volume/h
- ☐ A plan for at least 6 discus (not 2 or 3)
- ☐ Budget that covers the minimum group of discus (see the Romania estimate below)
- ☐ The tank is fully biologically cycled
- ☐ A long-term plan of 50% × 2/week water changes
🇷🇴 Romania estimate (2026): a minimum group of 6 discus ~1,200–2,400 RON (6 × ~200–400 RON each). Prices vary widely by country and variety — use your local market as the reference.
What to do next
📚 Learn more:
- Discus aquarium size — details by dimensions
- Discus-only vs community aquarium
- Community tank stocking
- Discus aggression: hierarchy vs bullying — why at least 6
- Discus aquarium filter
🧮 Calculate:
- Stocking calculator — for your exact volume
- Aquarium volume calculator — check the real capacity
- Water change calculator — frequency for your density
🛒 Buy:
- Verified discus — uniform groups of 6+