Corydoras

The corydoras (Corydoras spp.) is the roster's benthic detritivore — the only fish in V1 that grazes inside the tank rather than eating only the keeper's feed. It is a small, peaceful, armoured catfish (the genus is huge; C. aeneus and C. paleatus are the common aquarium species) that lives on the bottom of South American streams and floodplains, where it constantly sifts the substrate for food, taking it in with sensory barbels and passing mouthfuls of sand through its gills.

In EcoSym a fish is a boundary condition: biomass is the stocking level (fixed — no growth or breeding in V1), and the model reports its bioload and its health ∈ [0, 1].

In-tank grazing — the benthic difference

Unlike the four water-column fish (which eat the prepared feed only), the corydoras additionally reads in-tank food on the normal consumer feeding pathway: settled detritus (its primary in-tank food, sifted from the substrate) and incidental periphyton taken while foraging. It is not a specialised algae scraper — it has no radula — so its detritus preference is high but its assimilation is low (most detrital carbon is refractory or microbially bound, as for the amphipod shredder), and most biofilm-embedded algae stays refuge-protected from it.

Mechanically this matters because in-tank grazing is internal recycling, not a new mass source: whatever the cory eats from the tank reduces the additional feed the system needs, captured naturally by its appetite being satisfied partly from the substrate. In scenarios/fish_cory_benthic_grazing.yaml a small cory group visibly draws a standing stock of settled detritus down while staying at full health — the proof that the benthic food path works for a fish-category animal. (The cory does not, however, replace a clean-up crew or excuse overfeeding: uneaten food and feces still mineralize to ammonia.)

Tolerances and care notes

The corydoras is at the tolerant end of the roster for ammonia — alongside the betta, and above the zebra danio and the guppy. That is not where hobby lore usually puts a cory, and it is not where this simulator used to put one either: the model long carried the corydoras as its second-most sensitive fish, until the one controlled multi-species comparison that exists was read properly. In it, Corydoras came out the most ammonia-tolerant of eleven Amazonian species — the authors attribute it to the fish's facultative air-breathing — and the model's old value sat more than six times below that measurement. It prefers the cooler end of the tropical range (most common species do best around 22–26 °C). Two husbandry realities are reflected in the model: as a scaleless armoured catfish it is more sensitive to copper and many medications than scaled fish (a tighter copper-toxicity threshold), and as a bottom-dweller it has more contact with sediment hydrogen sulfide. Real corydoras supplement their oxygen by darting to the surface to gulp air (intestinal respiration), and the model now grants them that trait — as it does every catfish and loach family known for it, not the betta alone.

Diet and bioload

On top of its benthic grazing the cory eats the prepared feed (especially sinking pellets and wafers, and feed that reaches the substrate; preference 1.0, ~80 % assimilation). Its assimilated nitrogen is excreted as ammonia and the egested remainder — including its large, sinking feces — mineralizes to ammonia, the standard fish bioload chain.