Corydoras Catfish Stocking

Keeping Corydoras Catfish

The Corydoras Catfish grows to about 2.5 inches and needs a minimum of 20 gallons. Its bioload is low, meaning it produces a small amount of waste for its size, a key factor the "one inch per gallon" rule ignores. It's a schooling species, so keep at least 6 together or they'll be stressed. Temperament is peaceful, making it a good community fish. Use the calculator above to see how a group fits your tank's real volume before you buy.

How many corydoras in a 20 gallon tank?

Six of a standard-size species is the usual answer, and the calculator agrees with room to spare. A group of six bronze or albino cories at 2.5 inches lands at 44% of a 20-gallon tank's capacity in the calculator above, which leaves most of the tank for a midwater school: add eight neon tetras and the same tank reads 79%, still inside the comfortable band. That planted 20 long, six cories and a school of small tetras, is the most common stable community in the hobby, and it is worked through plan by plan in how many fish fit in a 20 gallon tank. On their own, up to 11 bronze cories stay comfortable in a 20. The answer changes a lot with the species, because "corydoras" covers well over a hundred fish that run from one inch to three.

Peppered cories are the edge case. At about 3 inches each they take a fifth more capacity per fish, so six of them read 53% of a 20 and 37% of a 29, and they are a cool-water fish that spends more of the day on the move than a bronze does. If you want peppered cories plus a full midwater school, the 29 gallon is the more honest tank.

Floor space matters more than gallons

Corydoras live on the bottom and forage across it in a loose group, so the footprint is the number to shop by. The 20 gallon is sold in two shapes: the 20 high at 24" × 12" and the 20 long at 30" × 12". Same water, but the long gives them 360 square inches of floor against 288, about 25% more, and the extra height of the 20 high is space cories never use. Pick the long.

Substrate is the other half of the floor. Cories root through the bottom with their barbels, and sharp or coarse gravel wears them down. Fine sand is ideal; smooth, small rounded gravel is acceptable. Keep the top of the substrate clean, since they spend their day with their face in it.

Group size and mixing species

Keep at least 6 of one species. Cories shoal most readily with their own kind, and a group of three different species is really three lonely fish. If you want variety, run two groups of six in a larger tank rather than one mixed handful in a small one. Bigger groups behave better: they spend more time in the open and less time hiding.

They are peaceful with almost any community fish that will not eat them. Avoid large cichlids and anything that nips, and check the temperature: most cories want 72 to 78 °F, which rules out some warm-water setups. Sterbai cories are the usual exception and tolerate the low 80s, which is why they are the standard choice alongside discus.

Feeding and the cleanup crew myth

Cories are sold as cleaners and they are not. They need their own sinking food, pellets or wafers that reach the bottom before the midwater fish finish it, and they feed best shortly after the lights go out. A cory living on leftovers is a slowly starving cory. Their occasional dash to the surface for a gulp of air is normal; they can breathe through their gut. Constant surface dashing is not, and usually points to poor oxygen or rising ammonia, so test the water.

They are also more sensitive to nitrate and to salt than their tough reputation suggests. Skip aquarium salt as a routine additive, and do not add a group to a tank that has not finished cycling.