Corydoras Catfish: Schooling, Substrate, and Diet
1) Schooling Behavior: Group Size, Species Choice, and Day–Night Rhythm
Corydoras catfish are social bottom dwellers that thrive in groups. In the wild they sift sandy stream beds in shoals, communicating with subtle body flicks and whisker contact. In aquaria, keeping them in true groups—not pairs—is the single best lever for welfare and natural behavior.
How many make a “school”? Aim for at least 8–10 of the same species for confident, visible behavior; 12–20 looks spectacular in longer tanks. Small groups hide more, breathe faster, and participate less in daytime foraging. Larger groups distribute social pressure so timid fish still feed, and the whole school cruises in a relaxed rhythm.
Species choice matters. Popular small species (e.g., C. panda, C. habrosus, C. pygmaeus, C. trilineatus) do great in community tanks. Larger or deeper‑bodied species (e.g., C. aeneus, C. sterbai) need more footprint but reward you with bolder presence. Pygmy and habrosus are micro‑schoolers that also mid‑water shoal; they still want groups of 10+ but can work in 10–15‑gallon planted tanks if scaped smartly. Avoid mixing several Cory species in a single small tank; schools fragment and you never see the cohesive behavior that makes them special.
Day–night rhythm. Corydoras are crepuscular: they feed and explore most at dawn and dusk. Provide dim ramps at lights on/off or floaters that soften the overhead glare; you’ll observe more relaxed foraging and synchronized turns. Midday they often rest in clusters under wood or plant shade, “periscope” breathing occasionally at the surface with their gut‑airing habit—normal behavior that helps them survive in warm, low‑O₂ backwaters.
Footprint > height. A 60–90 cm long tank gives room for line‑abreast foraging passes. Even “nano” corys appreciate width: a 60 × 30 cm footprint (≈ 20 gallon long) shows off group behavior better than a tall column. Keep return flow sweeping gently along the bottom so fine particles stay suspended until the filter captures them; corys dislike blasting currents that flip them or lift sand into their gills.
Companions & safety. Pair corys with small, calm mid‑water fish—rasboras, peaceful tetras, dwarf gouramis, and nano rainbowfish. Bottom territory bullies (large cichlids, loaches) or fin‑nippers stress corys and suppress feeding. Shrimp and snails are typically compatible. Always provide shaded “parking” spots under driftwood roots or broad crypt leaves where the group can rest undisturbed between foraging bouts.
2) Substrate & Layout: Sand vs Gravel, Leaf Litter, and Safe Décor
Substrate is everything for Corydoras. Their barbels (whiskers) are tactile organs for sifting sand; coarse or sharp gravel can erode barbels, invite infections, and permanently change how they feed. Give them a bottom they can push their faces into without injury.
Best choice: fine sand. 0.3–1 mm natural sand (pool filter sand, cosmetic sands, specialty aquarium sands) is ideal. The grains roll as corys “snorkel,” letting them jet sand out their gills while trapping edible micro‑bits. A 1–2 cm layer is plenty—deeper beds can work but need careful oxygenation and gentle stirring by inhabitants. If you prefer a planted layout with root‑hungry stems, consider sand front lanes with richer substrate or root tabs in the back (see root tab guide).
Acceptable compromise: small rounded gravel. If you must use gravel, choose rounded 1–3 mm pea types. Test by pressing the gravel into your palm—if it feels jagged, it’s too sharp. Rinse well to remove dust. Mix in leaf litter to soften the look and provide biofilm grazing.
Layout & leaf litter. Driftwood, rounded stones, and botanicals (catappa, oak leaves) create a natural benthic maze that slows flow and provides microfauna. Leaf litter is enrichment: corys root under and around it, and shrimp colonize it, creating a living buffet. Replace a handful of leaves monthly as they break down; pre‑soak to sink them and avoid floating clutter.
Filter intakes and safety. Use sponge pre‑filters on intakes to prevent barbels from contacting slotted plastic edges and to stop fine sand from abrading impellers. Place intakes 2–3 cm off the sand and point returns along the glass to keep currents laminar over the bottom rather than scouring.
Maintenance workflow. Vacuum lightly over sand, hovering to lift mulm rather than plunging like gravel. Stir a different section of sand each week with your fingers or a chopstick to prevent anaerobic pockets in deeper areas. During big re‑scapes, move the school to a quiet tub with a seasoned sponge filter so they’re not battered by cloudy water and tools.
3) Diet & Feeding: Sinking Foods, Live/Frozen, and Avoiding Overfeeding
“Bottom feeder” doesn’t mean “garbage disposal.” Corydoras are micro‑predators that hunt tiny invertebrates and biofilm. They will not “clean up” heavy flake dumps and, if you rely on leftovers, they end up under‑fed and stunted. Feed them directly with sinking foods sized to their mouths.
Staple foods. Use high‑quality sinking wafers and micro‑pellets with fishmeal/krill as the first ingredient and minimal fillers. Break larger wafers so everyone eats; in big groups, scatter food in multiple spots so dominant individuals don’t monopolize. At lights out, add extra micro‑pellets for timid fish that forage more in dim conditions.
Conditioning with live/frozen. Rotate frozen bloodworms (treat), daphnia, cyclops, and most of all live or freshly hatched baby brine shrimp for growth and spawning condition. Thaw frozen foods in tank water and swish to spread them; corys prefer punctuated “finds” over a single dump. Observe belly fullness: a gentle roundness after feeding is perfect—sunken bellies need more; bloated bellies mean you’re overdoing rich foods.
Feeding frequency. Two small feedings daily keep energy steady. In heavily planted tanks with biofilm and microfauna, once‑per‑day plus occasional live/frozen can suffice. Establish a “food call” by feeding in the same spots; within a week, your school will assemble quickly and everyone will get a portion.
Special considerations for pygmy/ dwarf Corydoras. C. pygmaeus and C. habrosus spend time mid‑water; offer some slow‑sinking micro‑foods (e.g., powdered pellets, crushed flakes) so food hangs in the water column where they naturally graze.
What to avoid. Don’t rely on algae wafers (corys aren’t grazers), don’t overuse fatty worms (risk of liver issues), and don’t expect them to survive on “leftovers.” Remove uneaten food after 10–15 minutes to protect water quality. Remember that fine sand hides debris—your test kit (nitrate trend) tells the truth about how much is accumulating.
4) Care & Health: Water Parameters, Filtration, and Common Issues
Clean, oxygen‑rich water keeps Corydoras thriving. They come from streams and floodplains that are soft, often slightly acidic, with abundant leaf litter and gentle flow. In the home aquarium, target stability and oxygenation, and give your filter easy jobs by keeping mechanical stages clean.
Parameters & stability. Most common Corydoras do well at 23–26 °C (73–79 °F), pH 6.2–7.5, and modest hardness. C. sterbai tolerates a bit warmer; pygmies often prefer the cooler end. Keep ammonia and nitrite at 0. Nitrate under 20–30 ppm is a good ceiling for long‑term fin and barbel health. If your tap is very soft, a touch of calcium/magnesium (GH) supports bones and egg development.
Filtration & flow. Provide broad, gentle flow via spray bars or lily pipes that sweep the bottom without kicking up sand. Use generous sponge or ceramic media for nitrification and rinse mechanical sponges weekly–bi‑weekly in old tank water. Add an air stone at night or during heat waves; oxygen saturation drops as temperature rises.
Common issues & prevention. Barbel erosion comes from sharp substrate, poor hygiene, or high organics—switch to sand, improve pre‑filter maintenance, and boost water changes. Red gills or surface gulping indicates low oxygen or ammonia/nitrite exposure—fix filtration/aeration first, then water changes. Ich and bacterial infections often hitchhike on new fish—quarantine newcomers 2–4 weeks, and treat in the hospital tank (see quarantine/hospital setup).
Breeding notes. Many Corydoras spawn after cool water changes that simulate rain. Condition the group with live/frozen foods for a few weeks, then perform a 30–50% change with slightly cooler, soft water while increasing flow/oxygenation. Look for the classic “T‑position” courtship and eggs placed on glass/plant leaves. Roll eggs to a hatching box with methylene blue if fungus threatens; feed fry infusoria then baby brine shrimp as they grow.
Maintenance rhythm. A simple weekly routine works: wipe glass, lightly hover‑vac the sand, rinse pre‑filter sponges, then perform a 30–40% water change with dechlorinated water roughly temperature‑matched. Log nitrates to gauge whether you’re removing enough waste; adjust feeding and filter cleaning accordingly.
FAQ
How many Corydoras should I keep?
Keep at least 8–10 of the same species; 12–20 in longer tanks looks and functions better. Larger groups distribute social pressure and bring out natural behavior.
Can Corydoras live on gravel?
Rounded 1–3 mm gravel can work, but fine sand is far safer for barbels and natural sifting. Avoid sharp, coarse gravel.
Do Corydoras clean the tank?
They help stir the bottom, but they are not janitors. Feed them directly with sinking foods; rely on water changes and filter maintenance for cleanliness.
Next reads: Choosing Substrate (Sand vs Gravel vs Soil) • Water Changes & Gravel Vacuum • Filter Media 101 • Hospital/Quarantine Tank • Beginner Plants
Labels: Corydoras, Catfish, Bottom Dwellers, Community Tanks, Beginner Guide