Guppy / Endler

The guppy (Poecilia reticulata, and its close relative the Endler) is the roster's hardy hard-water livebearer — a robust, adaptable community fish and one of the more pollution-tolerant aquarium species. It is a small poeciliid native to the hard, often brackish-influenced waters of northeastern South America and the Caribbean, and it has since naturalized across the tropics and subtropics worldwide (often deliberately, for mosquito control) — a global spread that is itself evidence of how tolerant it is.

In EcoSym a fish is a boundary condition: biomass is the stocking level (fixed in V1 — note that livebearer breeding is a deliberate post-V1 deferral, but the roster was chosen with it in mind), and the model reports its bioload and its health ∈ [0, 1].

A hardy, hard-water fish

The guppy's tolerances place it near the hardy end of the roster, just behind the zebra danio. Acute ammonia toxicity studies put its 96-hour LC50 at ~1.17 mg/L of unionized ammonia (Frances et al. 2023) — well above the neon tetra's lethal threshold — and it tolerates nitrite to ~9 mg/L NO₂-N in typical hard water (nitrite toxicity falls sharply as chloride rises, and guppy water is usually chloride-rich). Its one distinguishing preference is water chemistry: the guppy is a hard-water, high-GH fish that is comfortable in alkaline water and less happy in the soft, acidic water the neon prefers — the model gives it a pH comfort band shifted toward the alkaline side. This makes the guppy and the neon a natural illustration that "good water" is species-specific: the same tank suits one and stresses the other.

Diet and bioload

The guppy is a surface-and-mid-water omnivore. In the wild it grazes some aufwuchs, but in V1 it eats only the prepared feed the keeper adds (preference 1.0, ~80 % assimilation; in-tank algae grazing by a water-column fish is second-order). Its assimilated nitrogen is excreted as ammonia and the egested remainder mineralizes to ammonia — the standard fish bioload chain.