Zebra danio

The zebra danio (Danio rerio) is the hardiest fish in the roster and the species EcoSym recommends for fish-in cycling. It is a small (~3–4 cm), active, mid-water schooling cyprinid from the floodplains and streams of South Asia — a habitat that swings widely in temperature and water quality across the monsoon cycle, which is exactly why the fish is so tolerant. It is also the world's most-studied laboratory fish, so its physiology and toxicology are unusually well documented.

In EcoSym a fish is a boundary condition: its biomass is the number of fish you stock (held fixed — no growth or breeding in V1), and the model tracks the bioload it puts on the tank and its health ∈ [0, 1]. Every nitrogen atom the danio assimilates from its food is excreted as ammonia (it does not grow, so there is no somatic sink for nitrogen), which is what makes it a clean, mechanistic ammonia source for the cycling simulation.

Why it is the fish-in-cycling workhorse

The danio's tolerances are the widest in the roster: it is stressed only above ~0.06 mg/L of unionized ammonia (lethal ~0.4), tolerates nitrite up to ~10 mg/L NO₂-N, and survives temperatures from 10 to 38 °C. This is what lets it ride out the ammonia and nitrite spikes of a new tank's nitrogen cycle — its health dips during the spike and then climbs back toward 1.0 as the nitrifiers establish (see scenarios/fish_in_cycle_hardy_danio.yaml). That recoverable dip is the model's signature "the tank cycled and the fish were fine" story, and it is why a danio (or a similarly hardy fish) is the responsible choice for fish-in cycling — and why the fragile neon tetra is not.

Diet and bioload

The danio is a water-column feeder: in V1 it eats only the prepared feed the keeper adds (a planktonic danio nibbling biofilm is a second-order effect). It takes flake and small pellets avidly (preference 1.0), assimilates ~80 % of it, and excretes the assimilated nitrogen as ammonia while egesting the rest as feces that decay to detritus and mineralize to ammonia too — so the whole feed-nitrogen load eventually becomes ammonia, the bioload the tank must cycle.