Discus aquarium stocking: combinations for 200L, 300L, 400L
Stocking a discus community tank correctly — the 1 discus / 40–50L (10–13 US gal) formula, tested combinations for 200L, 300L, 400L, 500L+ and gradual-introduction rules.
Quick summary
- Base formula: 1 discus per 40–50L (10–13 US gal) (50L / 13 US gal recommended when planted).
- Schools of at least 10 for cardinals, rummynose, ember.
- Cory sterbai: 6 minimum. Ancistrus: 1–2 max.
- Gradual introduction: cory/pleco weeks 1–2, tetras weeks 3–4, discus week 5+.
- Real test: nitrates <20 ppm one week after a water change.
- Vertical balance: 3 zones (bottom / mid / surface).
In short
- Formula: 1 discus / 40–50L (10–13 US gal) (50L / 13 US gal for a planted tank).
- Schools of at least 10 for tetras (cardinals, rummynose, ember).
- Cory sterbai: at least 6. Ancistrus: 1–2 max (the males are territorial).
- Gradual introduction: cory/pleco weeks 1–2 → tetras weeks 3–4 → discus week 5+.
- Real load test: nitrates <20 ppm one week after a water change.
- Vertical balance: combine bottom + mid + surface fish.
What you'll learn
Stocking a discus (Symphysodon) community tank is a balance between aesthetics (a spectacular school) and biology (a load the filter and water quality can handle). Discus are extremely sensitive to nitrates — high density = competition at feeding time, a stressful hierarchy and Hexamita risk. This guide shows you the concrete formula, tested combinations for each volume and the critical rules of gradual introduction.
The base formula — how many fish fit
| Category | Rule |
|---|---|
| Discus | 1 fish / 40–50L (10–13 US gal) of water |
| Discus in a densely planted tank | 1 / 50L (13 US gal) |
| Schooling fish (cardinals, rummynose) | ~1 cm (0.4 in) of fish / 1L (0.26 US gal) |
| Cory sterbai | at least 6 (a school is mandatory) |
| Ancistrus / small pleco | 1–2 max |
🧮 Use the stocking calculator for an instant check on your volume.
Tested combinations by volume
🐠 200L (53 US gal) tank (100×50×40 cm / 39×20×16 in) — the absolute minimum
Reduced setup — discus only:
- 5 × adult discus (12–15 cm / 5–6 in)
Minimal community setup:
- 4 × discus
- 10 × neon cardinal
- 6 × Corydoras sterbai
⚠️ 200L (53 US gal) is the absolute minimum for discus. The community must be limited. Better to aim for 250L+ (66+ US gal) for flexibility.
🐠 250–300L (66–79 US gal) tank (120×50×50 cm / 47×20×20 in) — RECOMMENDED ⭐
Classic setup:
- 6 × adult discus
- 15 × neon cardinal
- 10 × rummynose tetra
- 6 × Corydoras sterbai
- 1 × Ancistrus bristlenose
Total: 38 fish — an excellent balance, nitrates <10 ppm with adequate filtration.
Alternative setup (Rio Negro biotope):
- 6 × discus
- 25 × neon cardinal (a large, spectacular school)
- 6 × Corydoras sterbai
- 1 × Ancistrus
🐠 400L (106 US gal) tank (150×60×50 cm / 59×24×20 in) — a rich community
Large setup:
- 8 × discus
- 20 × neon cardinal
- 15 × rummynose
- 10 × ember tetra
- 8 × Corydoras sterbai
- 2 × Ancistrus
- 1 × L134 Leopard Frog (optional, premium)
Total: 64 fish — needs powerful filtration (2× external filters or 1 sump + external filter, total flow 6× the volume/h).
🐠 500L+ (132+ US gal) tank (large) — maximum flexibility
Design freedom. Possible variants:
- 10–12 discus + large schools of 30+ cardinals
- 2 different tetra groups (cardinals + rummynose + ember = 40+ small fish)
- 2–3 different L-number plecos
- A school of 10–15 marbled hatchetfish for the surface zone
Important stocking rules
1. Gradual introduction is MANDATORY
NEVER introduce all the fish at once. The correct plan:
| Week | What you introduce | Why |
|---|---|---|
| 0 (cycling) | No fish | Complete biological cycling |
| 1–2 | Cory sterbai + Ancistrus | Hardy, establish the bacterial colony on the bottom |
| 3–4 | Tetras (cardinals, rummynose) | Schools settle in, a natural hierarchy forms |
| 5+ | Discus (LAST) | The most sensitive, they find stable water |
2. School density — critical
Tetras are absolute schooling fish. A small school = stress + nipping at the discus's fins.
| Species | Minimum school | Ideal school |
|---|---|---|
| Neon cardinal | 10 | 15–20 |
| Rummynose | 10 | 12–15 |
| Ember tetra | 15 | 20+ |
| Cory sterbai | 6 | 8+ |
| Lemon tetra | 8 | 10+ |
| Hatchetfish | 8 | 10+ |
3. Real biological-load test
Whatever the formulas say, test the nitrates as the final verdict:
- 1 week after a water change: nitrates < 10 ppm ✅ perfect
- 1 week after a water change: nitrates 10–20 ppm 🟡 acceptable
- 1 week after a water change: nitrates >20 ppm ❌ overstocked or weak filtration
4. Vertical balance — use every zone
| Zone | Suitable species |
|---|---|
| Surface | Hatchetfish (max 8 individuals, a tight-fitting lid!) |
| Mid-upper | Discus + cardinals + rummynose + ember |
| Bottom | Cory sterbai + Ancistrus + L-numbers |
Using all 3 zones = spectacular visibility + zero territorial competition.
5. Filtration in a dense community
- 200L (53 US gal) community: minimum flow 800–1200 L/h (210–315 US gal/h)
- 300L (79 US gal) community: minimum flow 1500–1800 L/h (400–475 US gal/h)
- 400L+ (106+ US gal) community: 2× external filters or a sump
A real example — a breeder in Romania (300L / 79 US gal)
| Species | Count | Zone |
|---|---|---|
| Discus Red Turquoise | 4 | Mid |
| Discus Pigeon Blood | 2 | Mid |
| Neon cardinal | 20 | Mid-upper |
| Rummynose tetra | 12 | Mid |
| Corydoras sterbai | 8 | Bottom |
| Ancistrus bristlenose | 1 | Bottom/decor |
Results after 12 months:
- Nitrates: 8 ppm one week after a 50% change
- Zero aggression
- Discus with spectacular colors, 14–16 cm (5.5–6.3 in)
- Water changes: 50% × 2/week
Common mistakes
❌ "I'll add 5 more neons, there's visual room" — Every fish raises the biological load. Test the nitrates first. ❌ "I'll buy 3 Cory to start" — Below 6 they're stressed and die in 3–6 months. 6 minimum from day one. ❌ "I'll put 2 male Ancistrus in 200L (53 US gal)" — Territoriality, fights. 1 male + 1 female or 2 females. ❌ "I'll introduce all the fish at once to save time" — An ammonia spike, mortality. Gradual is mandatory. ❌ "A school of 5 cardinals is enough" — Below 10 they nip the discus's fins. 10+ minimum.
Checklist before you stock
- ☐ The tank is fully cycled (ammonia + nitrites = 0 ppm)
- ☐ Enough volume for the 1 discus / 40–50L (10–13 US gal) formula
- ☐ A gradual-introduction plan over 5+ weeks
- ☐ Filtration with a flow of 4–6× the volume/hour
- ☐ Minimum schools respected (10 tetras, 6 Cory)
- ☐ A quarantine for each new species (4 weeks)
- ☐ A weekly nitrate check after each introduction
- ☐ A plan B: a 50L+ (13+ US gal) quarantine tank for emergency isolation
What to do next
📚 Learn more:
- Best discus tank mates
- Bottom feeders for discus
- Discus aggression and hierarchy
- Cycling the aquarium
- Filtration for discus
🧮 Calculate:
- Stocking calculator — check the load
- Aquarium volume calculator — confirm capacity
- Water change calculator — the optimal frequency
🛒 Buy:
- Verified breeders — safe sources
- Water tests — nitrate monitoring