Corydoras catfish care

Schooling: 6 minimum, same species

Corydoras are obligate schooling fish. In the wild they move in groups of dozens, foraging across river bottoms together. A solo cory hides under decor and rarely emerges; a pair is marginally better but still suppressed. The school behavior — tight grouping while foraging, synchronized air-gulps, coordinated rest — only shows up with 6+ of the same species.

The species-specificity is the catch most beginners miss. Buying 3 pandas and 3 sterbai because the store had them on sale produces two suppressed groups of 3, not one happy group of 6. Pick a species and commit; mixed-species tanks should run separate full schools (12+ corys total) which most community tanks can't accommodate.

Common species at-a-glance

The species most beginners encounter, with what they actually need:

• Bronze cory (C. aeneus) — the most common species in the trade. ~2.5 inches; hardy; tolerates the widest parameter range.
• Peppered cory (C. paleatus) — also very common. ~2.5 inches; tolerates slightly cooler water (down to 68°F), making it one of the few corys for unheated tanks.
• Panda cory (C. panda) — ~2 inches; distinctive black-and-white coloration. Slightly more sensitive to water-quality issues than the bronze.
• Sterbai cory (C. sterbai) — ~2.5 inches; the warm-water cory. Tolerates 78–82°F, making it the cory of choice for angelfish or discus tanks where most cories wouldn't handle the temperature.
• Pygmy cory (C. pygmaeus), habrosus, and hastatus— under 1.5 inches; can be kept in 10-gallon nano tanks. These three are sometimes called "dwarf corys" collectively.
• Julii cory (C. julii) — frequently mis-sold. The fish labeled "julii" in 95%+ of pet stores is actually C. trilineatus (the three-line cory). Both work the same in care; just know what you actually have.

Substrate: sand, not sharp gravel

Corys constantly sift the substrate with their barbels — the whisker-like sensory organs around the mouth. Sharp-edged gravel erodes those barbels over time, sometimes permanently. Eroded- barbel corys lose foraging efficiency, eat poorly, and become disease-prone. Mainstream consensus has shifted firmly: sand or smooth fine gravel is the substrate for corys.

• Pool filter sand (~$10/50 lb at hardware stores) — the cheap, effective option. Beige/tan color.
• Play sand — also cheap; rinse thoroughly before adding (cleaning out the silt takes longer than pool filter sand).
• Aquarium-specific sand — Caribsea Super Naturals and similar brands. More expensive, less rinsing required.
• Smooth pea gravel— acceptable; the rounded shape is gentle on barbels. Avoid angular "decorative" gravels.

What to avoid: sharp-edged decorative gravel, crushed coral (alkaline + sharp — a double mistake for corys), substrate with colored coatings (some leach into water).

Water parameters

Most cories tolerate a wider parameter range than smaller tetras. The headline exception is temperature — most cories prefer cooler water than the standard tropical-community range, and sterbai is the species you pick if you're running a warmer tank.

• Temperature: 72–78°F for most species. Sterbai handles 78–82°F. Peppered tolerates down to 68°F.
• pH: 6.0–7.5. Adaptable.
• GH: 5–12 dGH. Soft to moderately hard.
• KH: 2–8 dKH. Stable buffering.
• Ammonia / nitrite: 0 ppm sustained. Corys are mid-tier on water-quality sensitivity — more tolerant than cardinal tetras, less than zebrafish.
• Nitrate:< 20 ppm. Sustained higher nitrate contributes to barbel erosion and bacterial infections.

Diet: their own food, not table scraps

The most persistent corydoras myth: that they're a "cleanup crew" living on flakes that drift to the bottom. They're not. Most flake food gets eaten in the water column before reaching the substrate; what does fall is dispersed, dirty, and often not the right form factor for cory mouths. Corys need their own dedicated food.

• Sinking pellets / wafers as the staple. Hikari Algae Wafers, Repashy Soilent Green or Bottom Scratcher, Aquarium Co-Op Catfish Pellets. Drop one wafer per 4–6 corys once daily.
• Frozen bloodworms or brine shrimp1–2× per week. Broken into small pieces; corys aren't aggressive feeders and lose out to faster fish if pieces are too large.
• Blanched vegetables(zucchini, cucumber) occasionally. They'll graze if they find them.
• Feed at lights-out for tanks with aggressive top-feeders — ensures corys actually get the sinking food before tetras and barbs intercept it.

Tank mates

Corys are universally peaceful and bottom-oriented, which makes them compatible with most community fish. Good options:

• Tetras (any species) — share the mid-water column without competing
• Rasboras (harlequin, chili, espei) — peaceful, similar parameter range
• Dwarf gouramis, honey gouramis — peaceful surface-dwellers
• Otocinclus — algae-eaters; share the bottom but don't compete with corys for food
• Cherry / amano shrimp — peaceful coexistence; corys don't actively hunt shrimp
• Bettas (selected individuals) — most bettas tolerate corys; some don't. Test with a single cory before adding the school
• Angelfish — natural co-habitants in the wild; sterbai cory is the temperature match

Avoid:

• Aggressive cichlids (large American cichlids, most African cichlids) — will harass or kill corys
• Tiger barbs / serpae tetras — fin-nippers; corys have soft trailing fins and barbels that are vulnerable
• Goldfish — temperature mismatch (cool-water fish; corys are tropical)
• Other bottom-dwellers in the same niche (loaches larger than kuhli, plecos in small tanks) — territorial overlap

Common health issues

• Barbel erosion. Shortened or missing barbels. Cause: sharp substrate or sustained high nitrate. Fix: replace substrate with sand; reduce nitrate. Mild erosion can recover; severe erosion is permanent.
• Bacterial infections. Red streaks on the body, ulcers, lethargy. Usually water-quality-driven. Test water; do a 30% change; treat with a broad-spectrum antibiotic if advanced.
• Ich. White spots like grains of salt. Common after temperature drops or new-fish stress. Raise tank temp to 82°F for 10 days; treat with ich medication if persistent. Note: corys are sensitive to copper-based ich treatments — read package directions for cory-safe formulations.
• Internal parasites. Wild-caught corys occasionally arrive with internal parasites; tank-bred lines are usually clean. Signs: weight loss despite eating, white stringy feces. Treatment: praziquantel-medicated food per package.
• Stuck-in-decor death. Curious cory deaths from getting wedged in narrow decor gaps or filter intakes. Use sponge intake covers; avoid decor with narrow vertical crevices.

Where hobbyists disagree

• Sand vs gravel debate.Mostly settled in favor of sand, but some keepers still run smooth gravel without visible barbel issues. The conservative beginner-safe answer is sand; gravel only if it's explicitly smooth pea gravel.
• Mixed-species "schools." Some keepers report mixed cory groups schooling cross-species, especially among the dwarf species (pygmy, habrosus, hastatus). Mainstream consensus is still 6+ of one species.
• Pygmy cory placement. Marketed as bottom- dwellers, pygmy cories (especially C. hastatus) actually swim mid-water for substantial time — closer to a tetra in behavior than a typical cory. Don't expect the classic cory bottom-foraging look from them.
• How many is "enough". Some sources say 4 corys is workable. Modern consensus has shifted to 6 as the minimum, with 8–10 noticeably better for behavioral display.

Frequently asked questions

How many corydoras catfish should I keep together?

At minimum 6, of the same species. Corydoras are obligate schooling fish — solo or paired corys hide constantly, eat poorly, and miss out on the social behaviors that make them entertaining to keep. The common beginner mistake is buying 2 panda cories and 2 bronze cories thinking they'll school together; they won't, because schooling is species-specific. Pick one species and commit to a real group.

Sand or gravel for corydoras?

Sand. Mainstream consensus has shifted firmly toward sand over the last decade, and the reason is mechanical: corys constantly sift substrate with their delicate barbels (the whisker-like sensory organs around the mouth). Sharp-edged gravel cuts and erodes barbels over time, often permanently — eroded-barbel corys can't find food efficiently and are more disease-prone. Pool filter sand, play sand, or aquarium-specific fine sand all work. Smooth pea gravel is acceptable; standard angular gravel is not.

Why does my corydoras keep darting to the surface?

Normal behavior. Corydoras have an accessory air-breathing organ (a modified intestine that absorbs atmospheric oxygen) and periodically gulp air from the surface. A single quick dart-up-and-back-down every few minutes is healthy. What's NOT normal: gulping rapidly and constantly while staying near the surface — that's a sign of low dissolved oxygen or water-quality problems. Test ammonia and nitrite if you see frantic surfacing.

Can I mix different corydoras species in the same tank?

Technically yes, behaviorally no. Mixed-species cory groups will coexist peacefully but each species will keep to its own group, which means you've effectively created two small schools instead of one large one. The school behavior you're trying to see (tight grouping, coordinated foraging) only emerges with 6+ of the same species. The exception is pygmy/habrosus corys, which are sometimes reported to school across the dwarf-cory species — but mainstream consensus is still 6+ of one species.

Are corydoras good for beginners?

Yes, with two caveats. They're peaceful, hardy within their parameter range, and one of the most beginner-friendly schooling species. The caveats: they need correct substrate (sand), and they need their own sinking food — they're not a 'cleanup crew' that lives on tank scraps. The 'corys eat what falls to the bottom' claim is a myth that leads to slowly-starving cory schools. Sinking pellets or wafers (Hikari Algae Wafers, Repashy gels) once a day, plus the same staple flakes that drift down, keeps them well-fed.

Related

• Tank volume calculator — verify your tank actually holds 20+ gallons for a real cory school.
• Substrate calculator — figure out how much sand to buy for a 20-long.
• 20-gallon community setup — the standard tank for a cory school plus tank mates.
• Water parameters explained — nitrate management is the avoidable factor in barbel erosion.

Sources: FishBase (Corydoras genus pages), Aquarium Co-Op care references, peer-reviewed literature on Corydoras barbel anatomy and substrate effects (Reis 1998, Britto 2003), and mainstream hobby consensus on cory schooling and care. Where sources diverged, this guide takes the conservative beginner-safe position.