Corydoras tank size comes down to two numbers: six same-species fish is the working floor for a 20 gallon tank, and 60 cm of tank length is what binds. Our complete corydoras species profile covers the rest of the species; this page is the size decision.
| Corydoras tank size question | What the published sources say |
|---|---|
| How many corydoras in a 20 gallon tank? | 6 (Aquarium Co-Op) · 5 or more (FishBase) · 4 to 6 or more (Wikipedia) · 3 or more (PFK) |
| Minimum tank size | 20 US gallons for most varieties (Aquarium Co-Op); 60 cm length for 5+ (FishBase) |
| Maximum in 20 gallons | No sourced number — measure your tank’s base |
The Short Answer: How Many Corydoras Fit in a 20 Gallon Tank?
Four sources answer the headline question, disagreeing on one number: the smallest group worth keeping.
Six corydoras is the number the sources keep repeating
Aquarium Co-Op recommends “a group of six corydoras or more (all of the same species)”. For scale, wild groups run “from 20 to hundreds of the same species”; FishBase records adults in schools of 20 to 30. One or two corydoras isn’t a smaller school.
The 60 cm tank length is the rule that actually binds
FishBase’s aquarium keeping field for Corydoras aeneus and Corydoras paleatus puts two numbers in one sentence: groups of 5 or more, and a minimum aquarium size of 60 cm. Two tanks holding the same 20 gallons offer very different floor space.
What the Sources Actually Say About Corydoras Tank Size
Four published positions, each from a different kind of source.
FishBase: a 60 cm minimum aquarium for a group of five or more
FishBase is a species database, and its Corydoras aeneus and Corydoras paleatus entries carry the same aquarium keeping line: groups of 5 or more, minimum aquarium size 60 cm. The habitat ranges differ too, which is why a genus is not one animal.
Aquarium Co-Op: 20 gallons or more for most varieties
Aquarium Co-Op’s corydoras guide is retailer editorial, not a database, but it aims at tank buyers. It writes: “For dwarf species, a 10-gallon aquarium may be suitable, but we recommend 20 gallons or more for most other varieties.” The same page sets 72 to 82°F, pH 6.5 to 7.8, and smooth sand or gravel as the substrate.
Where the sources disagree, and what that means for a 20-gallon tank
| Source | Smallest group | Tank requirement |
|---|---|---|
| Aquarium Co-Op (retailer, 2026) | 6 of one species | 20 US gallons; 10 for dwarf species |
| FishBase (database, 2026) | 5 or more | 60 cm tank length |
| Practical Fishkeeping (magazine, 2026) | At least 3 | No tank size given |
| Wikipedia (encyclopedia, 2026) | 4 to 6 or more | No tank size given |
All four point the same way: 20 gallons is the starting point most sources support, and none calls it too small.
The only disagreement is the smallest group, and averaging those numbers would produce a figure no source supports, so we don’t. Take the strictest one you can afford, which is six.
Why there is no sourced upper limit, and what to do instead
Check the five published sources and you’ll find floors, never a ceiling: none states how many corydoras a 20 gallon tank holds at maximum. We won’t invent one. Measure the tank you own, keep the group at the sourced floor, and if you want more corydoras, add water.
Tank Shape: Why Long Beats Tall for Corydoras
Same 20 gallons, two very different tanks.
Bottom dwellers care about footprint first
Corydoras live on the bottom and sift substrate for food, so what they use is floor length, not depth. FishBase states the constraint in centimetres of length, not litres of volume, and Practical Fishkeeping describes a long tank where a group spawned regularly. Buy length, not height.
20-gallon long vs 20-gallon tall
A 20 gallon long tank spreads the same volume across a longer base; a tall tank stacks it upward. Bottom dwellers don’t use vertical space, so the tall version hands your corydoras less floor.
Check the inner length yourself: no source we could verify publishes standard retail dimensions. Our bristlenose pleco tank size guide runs the same logic.
Which Tank Size to Buy: 10, 15, or 20 Gallon
Choosing a tank size is a shelf decision, and one shelf has no published backing.
Measure your own tank in under a minute
Measure the inside length and inside width, not the outer frame: glass and rim eat into the base. If the inner length is under 60 cm, the tank doesn’t meet FishBase’s group minimum; your options are a longer tank or the dwarf species the 10-gallon branch covers. Measure before you buy, and the size question answers itself.
The liter conversions behind every tank size
NIST Special Publication 811 gives 1 US gallon as 3.785412 litres, so the table below converts every size from one standard. It also gives 1 inch as 2.54 cm. An 18 by 12 inch base is therefore 45.72 by 30.48 cm, the breeding footprint Wikipedia’s Corydoras sterbai entry recommends.
What each tank size actually supports
| Tank | Volume | 60 cm length met? | Group floor | Upper limit |
|---|---|---|---|---|
| 10 US gal | 37.85412 L | Your inner length | Dwarf species only | No source |
| 15 US gal | 56.78118 L | Your inner length | No source | No source |
| 20 US gal | 75.70824 L | Your inner length | 6, 5+, 4 to 6+, 3+ | No source |
Aquarium Co-Op puts the 10 gallon branch on dwarf species only. Twenty gallons is the shelf with the most published support; the 15 gallon shelf has none.
Before you fill a new tank, leak-test it: our guide to fixing a leaking fish tank starts with a dry test.
Stocking a 20-Gallon Corydoras Tank
Right tank size, wrong stocking, and you’re back at square one.
Buy the whole group at once
Every source agrees on the shape of the group even when the number differs. Six or more of one species, or 5 or more, or at least three while more is better.
Buy one species in one purchase; similarly shaped species still shoal separately. The first stocking mistake is shopping two fish now and four later.
Add the group together, then add the community
Move the whole group in at once after quarantine, not one fish at a time. Aquarium Co-Op sketches a 20 gallon community as a school of cory catfish, a school of small tetras and a centrepiece such as a honey gourami.
If the top slot is a betta, plan tank and neighbours together. Our betta tank size guide covers the other half, and bettas and shrimp in one tank the mixing rules.
Keeping the Group Healthy in a 20-Gallon Tank
The size decision is settled; now four things keep a school of six alive.
Water parameters to hold steady
Aquarium Co-Op gives 72 to 82°F and pH 6.5 to 7.8. Wikipedia’s genus entry gives 72 to 78°F and pH between 7 and 8, though that pH sentence carries a citation-needed tag. FishBase records habitat ranges per species: 25 to 28°C for Corydoras aeneus, 18 to 23°C for Corydoras paleatus, 21 to 25°C for Corydoras sterbai.
FishBase lists pH 6.0 to 8.0 for all three, and dH 5 to 19 for Corydoras aeneus and Corydoras paleatus against 2 to 25 for Corydoras sterbai. Mix species and you align the ranges first.
Our aquarium water parameters guide covers the seven measurements; the fish fungus guide covers the white patches.
Flow, substrate, and hiding places
Aquarium Co-Op states that smooth sand or gravel is preferred, and that’s where the group wants to be. Give them open sand to work over, plus driftwood or a cave to hide behind.
No source we could verify gives a substrate depth, flow rate or water change percentage, so measure them yourself. Give the group cover at the bottom, keep the middle open for swimming, and start with our beginner-friendly aquarium plants list.
Feeding a group of bottom-dwellers
Practical Fishkeeping describes the routine as feeding a sinking catfish tablet daily, so one sinking feed a day aimed at the bottom where the group feeds. No published source gives a per-fish portion, so don’t count pellets per fish; watch the group instead. Two references worth keeping: what neon tetras eat, and can bettas eat bloodworms on frozen, live and freeze-dried bloodworms.
Sources
- FishBase — Corydoras aeneus — authority, 2026
- FishBase — Corydoras paleatus — authority, 2026
- FishBase — Corydoras sterbai — authority, 2026
- Aquarium Co-Op — Corydoras guide — retailer editorial, 2026
- Practical Fishkeeping — Corydoras — magazine, 2026
- Wikipedia — Corydoras — encyclopedia, 2026
- Wikipedia — Corydoras aeneus — encyclopedia, 2026
- Wikipedia — Corydoras sterbai — encyclopedia, 2026
- NIST Special Publication 811 — government standard, 2026
- Photo: “Corydoras aeneus 1.0.JPG” by Jofre (Public domain) — https://commons.wikimedia.org/wiki/File:Corydoras_aeneus_1.0.JPG; “Corydoras aeneus 5.jpg” by Jofre (Public domain) — https://commons.wikimedia.org/wiki/File:Corydoras_aeneus_5.jpg; “Corydoras aeneus 3.0.JPG” by Jofre (Public domain) — https://commons.wikimedia.org/wiki/File:Corydoras_aeneus_3.0.JPG; retrieved 2026-11-03
