Surface fish for discus: hatchetfish, pencilfish

In short

• The surface zone is the most neglected one in a discus aquarium — discus stay in the middle.
• Top: marbled hatchetfish (Carnegiella strigata) — school of at least 8.
• Alternative: pencilfish (Nannostomus eques, N. marginatus) — school of at least 6.
• A TIGHT-FITTING LID IS MANDATORY — all surface fish jump out of the tank.
• Killifish (Aphyosemion): for enthusiasts — short lifespan of 1–3 years.
• Tank of at least 200L (53 US gal) with a calm surface (filtration without big waves).

What you'll learn

In a discus aquarium, the surface zone often stays empty — discus (Symphysodon) stay in the middle, cardinals in the mid-upper level, Cory on the bottom. Adding a school of surface fish rounds out the tank vertically and creates a three-dimensional dynamic. This guide shows you the 3 compatible groups at 29°C (84°F) and the critical rules (especially the tight-fitting lid, ignored by 90% of beginners).

Why add surface fish to a discus aquarium

✅ They use the free zone — discus don't constantly come to the surface ✅ A more dynamic tank — movement in all 3 zones ✅ They eat insects that land on the surface (mosquitoes, flies) — natural control ✅ Unique behavior — hatchetfish "fly" (briefly leave the water for insects) ✅ Zero competition with the discus for food (they feed in a different zone)

Top options — species by species

1. Marbled hatchetfish (Carnegiella strigata) ⭐ #1

Native to the Amazon and Orinoco — a natural biotope alongside discus.

Appearance: a laterally compressed body with a hatchet shape (hence "hatchet"), black/silver marbled patterns with a brown/golden patch on the back.

Unique behavior: they "fly" — they use their extended pectoral fins to leap 1–3 cm (0.4–1.2 in) above the water to catch insects. A tightly synchronized school.

⚠️ A TIGHT-FITTING LID IS ABSOLUTELY MANDATORY. They jump through:

• Gaps larger than 1 cm (0.4 in)
• Holes for filter cables
• The space between the lid and the tank rim

2. Common hatchetfish (Gasteropelecus sternicla) — a larger alternative

⚠️ 28°C (82°F) limit — stressed at a constant 29–30°C (84–86°F). Recommended only for tanks running at 28°C (82°F).

3. Pencilfish — the elegant alternative

Nannostomus eques (Diptail Pencilfish)

Quirk: the only species that swims diagonally at 45° with its head up. It stays in the surface and mid-upper zone.

Nannostomus marginatus (Dwarf Pencilfish)

Appearance: 3 horizontal black stripes on a yellow-orange body.

⚠️ 28°C (82°F) limit for both pencilfish — at 29°C (84°F) they tolerate it but don't thrive.

4. Killifish — for enthusiasts

Aphyosemion australe (Lyretail Killifish)

❌ NOT compatible with discus at 29°C (84°F). Annual or semi-annual killifish prefer lower temperatures. For dedicated tanks only, not with discus.

5. Florida flagfish (Jordanella floridae) ❌

Cool water (18–24°C / 64–75°F) + aggressive. Totally incompatible with discus.

6. Endler's livebearers ❌

Hard, alkaline water (pH 7.5+). Incompatible with the soft water discus need.

Comparison table

Critical rules for surface fish

Rule 1: A 100% tight-fitting lid ⚠️

The most common cause of death: jumping out of the tank.

Check:

• ☐ A lid with no gaps >1 cm (0.4 in)
• ☐ Filter-cable holes covered with film or mesh
• ☐ Space between the lid and the tank rim = 0
• ☐ On open-top tanks, fine mesh across the top

Test: approach the tank suddenly — the fish jump reflexively. If any clears the rim, the lid isn't sealed.

Rule 2: A calm surface

Surface fish can't handle big waves. Use filtration with a deflector or an outflow directed downward.

❌ An external filter with a jet aimed straight at the surface = permanent stress ✅ A filter with a horizontal spray bar below the surface = ideal

Rule 3: Floating plants help

Floating plants (Limnobium laevigatum — Frogbit, Salvinia, Pistia) provide:

• Hiding spots at the surface
• Shade over the zone (hatchetfish prefer low light)
• Natural biotope aesthetics

Rule 4: Surface feeding

Specific floating food:

• Small dried insects (Tetra Insect Cuisine)
• Frozen mosquito larvae kept at the surface
• Flakes that float 2–3 min before sinking

Don't wait for surface fish to feed off the bottom — they won't.

Recommended setups

250L (66 US gal) discus tank (minimum)

• 6 × discus (middle)
• 15 × cardinal tetra (mid-upper)
• 6 × Cory sterbai (bottom)
• 8 × marbled hatchetfish (surface) ⭐
• 1 × Ancistrus

400L (106 US gal) discus tank (deluxe)

• 8 × discus (middle)
• 20 × cardinal tetra + 15 rummynose (middle)
• 8 × Cory + 2 Ancistrus (bottom)
• 10 × marbled hatchetfish (surface)
• 6 × pencilfish marginatus (mid-upper)

Common mistakes

❌ "Hatchetfish won't jump in my small tank" — They jump until one gets out. A tight-fitting lid before purchase.

❌ "I'll get 3 hatchetfish and see how it goes" — Below 8 they're stressed, don't school and die within 1–3 months from stress. At least 8.

❌ "Killifish are small and pretty, they'll work with discus" — Cool water + short lifespan + incompatible parameters. NO.

❌ "Pencilfish at 29°C (84°F) with discus" — Maximum tolerance 28°C (82°F). At a constant 29°C they suffer. Hatchetfish are better.

❌ "An external-filter outflow aimed straight up = better oxygen" — It destroys the hatchetfish school with waves. A spray bar below the surface or an outflow aimed downward.

Gradual introduction plan

Surface fish are sensitive to stress — introduce them slowly:

Week 1:

• Drip acclimation, 2 hours
• Introduce only 4 hatchetfish (a tank test)
• Reduce light intensity

Week 2:

• Check that all 4 are healthy and eating
• Add the rest (4 or 6 — total 8–10)

Week 3+:

• Daily monitoring
• Dedicated surface insect feeding

Checklist before buying

• ☐ A tank of at least 200L (53 US gal)
• ☐ A tight-fitting lid verified (zero gaps >1 cm / 0.4 in)
• ☐ Cable holes covered with mesh
• ☐ Filtration with a calm surface (spray bar or downward outflow)
• ☐ Floating plants available (Frogbit/Salvinia)
• ☐ Specific floating food
• ☐ The minimum school respected (8 hatchetfish, 6 pencilfish)
• ☐ A 4-week quarantine available

What to do next

📚 Learn more:

• Best discus tank mates
• Tetra for discus — comparison
• Bottom feeders for discus
• How many discus in an aquarium
• Discus-compatible plants — includes floating plants

🧮 Calculate:

• Stocking calculator — total load

🛒 Buy:

• Floating food for surface fish
• Floating plants