Discus aquarium size: how many liters for how many fish
Discus aquarium size — a complete table with the minimum volume (150 L / ~40 US gal for 4 fish, 250 L / ~66 US gal for 6, 350 L / ~92 US gal for 8), the 45 cm (18 in) minimum height rule, the right L×W×H proportions and how to place the tank at home.
Quick summary
- Liters-per-fish rule: 40–50 L (~10.5–13 US gal) per adult discus (12–18 cm / 4.7–7 in at maturity).
- Practical minimum: 200 L (~53 US gal) for 4–6 discus — below ~150 L (~40 US gal) means chronic stress and bullying.
- Minimum height: 45–50 cm (18–20 in) — discus are tall, rounded fish.
- Prefer long & wide over tall & narrow — 150×60×50 cm (59×24×20 in) beats 100×40×70 cm (40×16×28 in).
- A lid is essential — discus jump when stressed or at night.
- Always plan for the weight: a 250 L (~66 US gal) tank with stand is roughly 300 kg (660 lb).
In short
- Liters-per-fish rule: 40–50 L (~10.5–13 US gal) per adult discus (12–18 cm / 4.7–7 in at maturity).
- Practical minimum: 200 L (~53 US gal) for 4–6 discus — below ~150 L (~40 US gal) = chronic stress + harmful bullying.
- Minimum height: 45–50 cm (18–20 in) (discus are rounded, tall-bodied fish).
- Prefer long & wide over tall & narrow — 150×60×50 cm (59×24×20 in) is better than 100×40×70 cm (40×16×28 in).
- A lid is essential — discus jump under stress or at night.
- Always plan for the weight — a 250 L (~66 US gal) tank with stand is roughly 300 kg (660 lb).
💱 This guide focuses on volumes, dimensions and proportions, which apply everywhere. Tank prices vary enormously by country, so we don't publish fixed international prices. Where Romania-specific figures appear, they are clearly labelled as a Romania estimate — replace them with your own local prices.
What you'll learn
Tank size is the first factor that decides whether discus (Symphysodon) thrive or live in chronic stress. Discus are large (12–18 cm / 4.7–7 in at maturity), gregarious, tall-bodied fish — they need volume, height and horizontal floor space to form natural hierarchies. This guide gives you the exact liters-per-fish table, recommended dimensions per group (4, 6, 8, 12 fish), the correct L×W×H ratio, placement rules at home and the mistakes to avoid.
Volume & dimensions table — by group size
| Number of discus | Minimum volume | Optimal dimensions (L×W×H) |
|---|---|---|
| 4 fish (minimum group) | 150 L (~40 US gal) | 100×50×40 cm (40×20×16 in) |
| 6 fish ⭐ recommended | 200–250 L (~53–66 US gal) | 120×50×45 cm (47×20×18 in) |
| 8 fish | 300–350 L (~79–92 US gal) | 130×55×50 cm (51×22×20 in) |
| 10 fish | 400 L (~106 US gal) | 150×60×50 cm (59×24×20 in) |
| 12+ fish | 500+ L (~132 US gal+) | 160×60×55 cm (63×24×22 in) |
| Breeding pair | 100–150 L (~26–40 US gal) | 80×45×45 cm (31×18×18 in) |
🧮 Work out exactly how many discus fit in your tank with the stocking calculator and the aquarium volume calculator — enter the dimensions, get an instant recommendation.
The 40–50 L (~10.5–13 US gal) per discus rule — explained
Discus reach 12–18 cm (4.7–7 in) at maturity (2–3 years) and can reach 22 cm (8.7 in) in rare varieties (wild Heckel). They need:
- Room for a hierarchy — each discus claims a territory roughly 20–30 cm (8–12 in) across
- Visual distance — subordinate fish need to be able to "disappear" from the dominant fish's line of sight
- Water stability — a larger volume means more stable parameters (ammonia and nitrite rise more slowly)
- Dissolved oxygen — a larger surface area means better gas exchange
The real math:
- 4 fish × 40 L = 160 L minimum → round up to 200 L (~53 US gal) for a safety margin
- 6 fish × 40 L = 240 L → a standard 250 L (~66 US gal) tank
- 8 fish × 40 L = 320 L → 350 L (~92 US gal)
❌ What does NOT work: 6 discus in 100 L (~26 US gal) "because they're juveniles" — within 6–8 months they grow into adults, and losses tend to appear exactly when you should be enjoying them.
Why height matters — 45–50 cm (18–20 in) minimum
Discus are rounded, tall, laterally compressed fish — their silhouette is vertical, not horizontal like many other cichlids.
| Aspect | Why it matters |
|---|---|
| Body shape | An adult is 10–15 cm (3.9–5.9 in) tall — it needs vertical swimming space |
| Natural behavior | They swim in the water column (mid-water), not on the bottom |
| Natural look | A 30–35 cm (12–14 in) tall tank makes them look "squashed" |
| Stress | Tanks that are too shallow can cause chronic stress and pale colors |
❌ Tanks that don't suit discus:
- "Long" aquascape-style tanks (60–80 cm / 24–31 in long × 30 cm / 12 in tall) — too shallow
- Cubes under 40 cm (16 in) tall — even if the volume is fine
✅ Recommended: standard rectangular tanks with an L:H ratio ≈ 2.5:1 (e.g. 120×50×45 cm / 47×20×18 in).
Long & wide vs tall & narrow — which is better
Compare two tanks of the same volume (250 L / ~66 US gal):
| Tank A | Tank B |
|---|---|
| 150×50×35 cm (59×20×14 in) — too shallow | 100×40×60 cm (40×16×24 in) — too tall and narrow |
| ❌ Below the minimum height | ❌ Reduced water surface |
| ❌ Discus cramped vertically | ❌ Less dissolved oxygen |
The best solution for 250 L (~66 US gal):
- 120×50×45 cm (47×20×18 in) — balanced proportions
- Or 130×55×45 cm (51×22×18 in) — even better for 6–8 fish
Why water surface matters:
- More surface = more O₂/CO₂ gas exchange
- More horizontal swimming space for a natural hierarchy
- Easier to keep parameters stable
A lid is essential — not optional
Discus jump. Not often, but enough to lose one to the floor in a single stressful night.
Situations that trigger jumping:
- A sudden parameter change (cold fresh water)
- Lights switched on abruptly at night
- Aggressive fish in the tank
- Loud noise / vibration
- Bullying within the group
Acceptable lid types:
- ✅ An integrated lid with LED (e.g. Juwel, Aquael) — the best option
- ✅ A sliding glass lid (leaving only a gap for the filter)
- ✅ A fine plastic mesh (DIY) — effective and cheap
❌ Open-top aquascape tanks are NOT suitable for discus (unless you keep the water level 5–8 cm / 2–3 in below the rim).
Placement at home — 5 rules
| # | Rule | Why |
|---|---|---|
| 1 | Away from direct windows | Temperature swings + algae from sunlight |
| 2 | Away from doors and heavy foot traffic | Discus startle at sudden movement |
| 3 | Away from noise sources (TV, speakers) | Chronic stress, food refusal |
| 4 | On solid furniture (250 L = ~280 kg / 617 lb of water alone!) | A standard 250 L tank + stand is over 300 kg (660 lb) |
| 5 | Easy access for water changes | You'll do 100+ changes a year — keep it near a tap |
Weight by volume:
- 100 L (~26 US gal) tank = ~110 kg (243 lb) with water + décor
- 250 L (~66 US gal) = ~280 kg (617 lb)
- 400 L (~106 US gal) = ~450 kg (992 lb)
- 500 L+ (~132 US gal+) = needs a concrete floor or a structural check
New vs second-hand tank — which to choose
New tank
✅ Pros:
- A 2–5 year manufacturer warranty
- Verified silicone seams
- Comes with instructions and support
Reliable global brands: Aquael (Poland), Juwel (Germany — premium), plus quality local manufacturers.
Second-hand tank
✅ Pros:
- 30–50% cheaper
- Often comes with a stand and equipment included
❌ Risks:
- Aged silicone — risk of cracking + a flood
- Glass with micro-scratches (persistent algae)
- Needs a 48h water-pressure test before stocking
Tip: if you go second-hand, re-seal it preventively (at a specialist workshop) or test it filled with water in a garage for 7 days first.
Common mistakes when choosing a tank
❌ "100 L (~26 US gal) is enough for 4 discus to start" — within 8–12 months they'll be adults, and moving them to a bigger tank means stress + two more acclimation cycles.
❌ "I'll get an 80 cm (31 in) tall tank, it looks impressive" — discus don't use height above ~50 cm (20 in). You lose usable volume up top and make maintenance harder.
❌ "A round / curved tank is prettier" — it distorts visibility, and discus get stressed by their own warped reflection.
❌ "My flat-pack cabinet can take the weight" — check the maximum load per m². A 250 L+ (66 US gal+) tank needs a dedicated stand or masonry.
❌ "I'll skip the lid and use floating plants" — plants won't stop a startled discus from jumping.
❌ "Under 200 L (~53 US gal) is fine with more frequent water changes" — overstocking causes behavioral stress that chemistry can't fix.
Final checklist — ready to buy the tank?
Tick each point before you pay:
- ☐ Volume confirmed for your target number of fish (40–50 L / ~10.5–13 US gal per adult)
- ☐ Dimensions: long & wide preferred (L:H ratio ≈ 2.5:1)
- ☐ Height at least 45 cm (18 in), ideally 45–55 cm (18–22 in)
- ☐ A lid included or prepared for a DIY lid
- ☐ A solid stand checked — maximum load per m²
- ☐ A spot identified at home (away from windows, doors, noise)
- ☐ Easy access for water changes (within ~5 m / 16 ft of a tap)
- ☐ Total budget checked — tank + stand + transport
What to do next
📚 Learn more: Essential discus aquarium equipment — filter, heater and tests for your chosen volume.
🧮 Calculate: aquarium volume calculator and stocking calculator — check exactly how many discus fit in your chosen dimensions.
🛒 Buy: Tanks and stands in the shop — verified models.
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