Compatibility & Tank Mates

Top 3: cardinal tetra (Paracheirodon axelrodi, NOT innesi), rummynose tetra (Hemigrammus bleheri) and Corydoras sterbai. For the bottom: Ancistrus bristlenose (max 15 cm). For the surface: marbled hatchetfish. All need schools of at least 10 (except the pleco). All tolerate 28–30°C long term.

The blacklist: tiger barb and serpae tetra (fin-nippers), guppy/molly/platy (hard water + Camallanus/TB), goldfish/koi (cold water), common pleco (attacks the slime coat), Oscar/Arowana (predators), angelfish (asymptomatic Hexamita), African Mbuna cichlids (pH 7.8+). No matter what the shop says.

Not recommended. Angelfish are asymptomatic carriers of Hexamita, Capillaria, Tetrahymena and Mycobacterium — all fatal to discus. Rare success cases: a 400L+ aquarium, angelfish raised from fry alongside the discus, a 6–8 week quarantine with preventive Metronidazole. Alternative: discus of contrasting varieties for a vertical visual effect.

ONLY Corydoras sterbai — the only species that tolerates 29–30°C long term. Aeneus, paleatus, julii, panda prefer 22–26°C and get sick within 3–6 months at discus temperatures. A minimum of 6 is mandatory — below that they're stressed and don't form a school. EU 2026 price: 8–15 €/piece.

The common pleco (Pterygoplichthys) has 2 problems: 1) it reaches 40–50 cm, 2) at night it latches onto the flanks of large fish and eats their slime coat → open wounds + secondary Columnaris. Use an Ancistrus bristlenose (max 15 cm) or premium L-numbers (L134 Leopard Frog, L200 Green Phantom, L201 Inspector).

Hierarchy (normal): the alpha comes first BUT everyone eats, subordinates retreat TEMPORARILY, all keep good colors. Bullying (pathological): one fish chased CONSTANTLY, not eating for 5+ days, hiding permanently, persistently dark colors, torn fins, visible weight loss. 2+ signs = intervene within 24–48h.

A minimum of 6 discus for a healthy hierarchy, ideally 8–10. Below 5, the natural aggression focuses on 1–2 targets and pathological bullying appears. Below 4, the dynamic is dysfunctional. Exception: a breeding pair in a dedicated 100L+ aquarium.

The tested 300L setup: 6 discus + 15 cardinal tetra + 10 rummynose + 6 Corydoras sterbai + 1 Ancistrus = 38 fish. The base formula: 1 discus per 40–50L. Check nitrates 1 week after a water change — below 10 ppm = perfect, above 20 ppm = overstocked.

Weeks 1–2 (after cycling): Cory sterbai + Ancistrus. Weeks 3–4: tetra (cardinals, rummynose). Week 5+: the DISCUS (LAST). Never all at once — an ammonia spike even in a cycled aquarium. A 4-week quarantine for each new species in a separate tank.

DISCUS ONLY for the first 6–12 months. The learning curve is already steep (parameters, feeding, diseases), quick problem diagnosis (all fish visible), a lower start-up budget (~50% reduction), and a future transition to a community is easy. Add tank mates AFTER 6+ months of stability with no problems.

The formula: 1 discus per 40–50L of real water (gross volume × 0.85). 200L gross = 4–5 discus max (the absolute minimum). 250L = 5–6 discus. 300L = 6–7 ideal. 400L = 8–10. Aquarium height at least 45 cm (discus are tall). A minimum of 6 discus per group for a healthy hierarchy. Understocking > overstocking.

The top 3 confirmed at 29°C: cardinal tetra (Paracheirodon axelrodi, NOT innesi!), rummynose (Hemigrammus bleheri, a red nose = a water-quality indicator), ember tetra (Hyphessobrycon amandae). Schools of at least 10 (cardinal/rummynose), 15 (ember). For a spectacular two-tone look: 15 cardinals + 10 rummynose. For an authentic biotope: a single school of 25 cardinals.

Marbled hatchetfish (Carnegiella strigata) — the only surface species that thrives at 29°C, a school of at least 8, 3–4 cm. Alternatives with reservations (they prefer 28°C): pencilfish (Nannostomus eques / marginatus). A TIGHT LID IS MANDATORY — hatchetfish jump through any gap >1 cm, including filter cable holes. Killifish are NOT compatible (they prefer 22–26°C).

YES, they're 100% compatible. Top picks: A. agassizii (the most popular, males with a spade tail), A. cacatuoides (cockatoo). A. borellii is marginal (the 27°C limit). Identical parameters: pH 6.0–7.0, 28–30°C. A territorial zone ONLY on the bottom — zero competition with the discus (mid-water). The rule: max 1 male per species. A minimum 250L aquarium with a cave + fine sand substrate.

Shrimp: ONLY Amano (Caridina multidentata, 4–5 cm) survive safely with adult discus. Adult cherry shrimp are POSSIBLE with dense Java Moss, but the babies get eaten. Crystal Red and Sulawesi are INCOMPATIBLE. Snails: Nerite (perfect, they don't breed in freshwater) and MTS (substrate aeration). AVOID Ramshorn and Pond snails (they breed explosively = a pest).

A 4-week quarantine is MANDATORY for ANY species. A cycled 50L+ quarantine aquarium. Drip acclimation of 1–2 hours (NOT 20 min). Week 2: Praziquantel (PraziPro/Sera Tremazol) + 0.1% salt preventively. Integration: rearrange the décor (reset territories) + lights off for 24h. THROW OUT the bag water. Risks avoided: Hexamita, Camallanus, Mycobacterium (all asymptomatic).

A 400L+ aquarium with lightly remineralized RO water (TDS 60 ppm, pH 6.0, KH 1). Décor: fine white sand + massive driftwood + catappa leaves + alder cones. Filtration 4–5× volume/h WITHOUT carbon (it absorbs the tannins). Low LED lighting + floating Frogbit plants. Native species: discus + cardinals + Apistogramma agassizii + Cory adolfoi/sterbai + Ancistrus + marbled hatchetfish. Budget: 4800–8700 RON.

Top 5 killers: an uncycled aquarium (70%), temperature below 28°C (50%), an undersized filter (40%), cheap sick fish (60%), preventive treatments without a diagnosis (30%).

The first year: discus ONLY. After 1 year, options: Corydoras sterbai, cardinal tetra (careful — they can be eaten). NEVER: angelfish (parasites), barbs (nip), guppies (different requirements).

NO. Treatments with no symptom = they destroy the filter's bacterial colony → ammonia crisis. Treat only based on confirmed symptoms. At the first sign: test the water (90% of 'diseases' are bad parameters).

Common invisible causes: ammonia below 0.25 ppm (undetectable with strips), temperature dropping at night, old oxidized food, a bad source (fish already sick at purchase).

9 dark vertical bars on the body, NORMALLY present but temporarily visible under stress. A problem only if they stay dark for over 48h with no cause (new lights, moving, feeding).

Probably a subordinate (bullied). Check whether it eats, stays alone in a top corner, with clamped fins. Solution: rearrange the décor, more discus (dilutes aggression), or temporary separation.

Weeks 1–2: stress, partial refusal of food. Weeks 3–4: hierarchy + colors appearing. Month 2: stable routine. Month 3: fully normal behavior. See the detailed calendar.

If it's temporary (someone walked through the room, lights just turned on): normal, they come back. If it's permanent: test the water immediately, check the temperature, watch the breathing.