Neon tetra

The neon tetra (Paracheirodon innesi) is the fragile fish of the roster — the cautionary "wrong fish, too soon" case. It is a small (~3 cm) shoaling characin from the soft, acidic, blackwater tributaries of the upper Amazon and Orinoco basins: water that is naturally low in minerals, low in dissolved ammonia (because the acidic pH keeps almost all of it as non-toxic ammonium), and biologically stable. A fish adapted to that constancy has little tolerance for the swings of a new tank.

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].

Why it is the cautionary fish

The neon's tolerance thresholds are the tightest in the roster: stress begins at just ~0.02 mg/L of unionized ammonia and is lethal by ~0.15, nitrite stress begins at ~0.3 mg/L NO₂-N, and it is more oxygen-demanding and more pH-sensitive than the hardy fish. These numbers come from tolerance studies on its close congener the cardinal tetra (P. axelrodi), which was the most ammonia-sensitive species in a multi-species comparison (Oliveira et al. 2008) — Amazonian blackwater characins are simply more sensitive to ammonia than most freshwater fish.

The consequence is dramatic in scenarios/fish_in_cycle_overstocked_neon.yaml: the same bare tank and bioload mechanism that a zebra danio rides out instead pushes the neon's health past the point of no return. As the new-tank ammonia and nitrite spike, neon health crashes (where the danio merely dipped), the population dies off, and the decaying fish produce a second ammonia bump — "the tank got worse after the fish died." It is the model's clearest demonstration that the fish you choose, and when you add it, matters more than anything else in fish-in cycling.

The flip side is the happy path: drop a light neon stock into an already-cycled tank and the mature nitrifiers keep ammonia and nitrite at zero, so even this fragile fish stays at full health. The neon is not a bad fish — it is a bad first fish for an uncycled tank.

Diet and bioload

A small mid-water shoaler, the neon eats only the prepared feed in V1 (preference 1.0, ~80 % assimilation). Its assimilated nitrogen is excreted as ammonia and its egested remainder mineralizes to ammonia — the same bioload chain as every fish, just from a fish far less able to survive that bioload going uncycled.