Taxonomy & naming
Microcharacidium eleotrioides was described by the French ichthyologist Jacques Géry in 1960. It belongs to the family Crenuchidae — the South American darters, which also includes the better-known sailfin characins Crenuchus and Poecilocharax — within the order Characiformes. The genus Microcharacidium is a small group of diminutive, bottom-associated darters distinguished from the free-swimming tetras most aquarists picture when they hear "characin."
The species epithet eleotrioides means "eleotrid-like" or "resembling a sleeper goby," a direct nod to its posture and behaviour: rather than schooling in open water, it perches on or near the substrate and moves in short darting bursts, much as gobies and sleeper gobies (family Eleotridae) do. That resemblance in habit, not any close relationship to true gobies, is the basis for the name.
Morphology
Microcharacidium eleotrioides is one of the smallest characins in the hobby, with FishBase recording a maximum size of just 1 in standard length. The body plan follows the general darter-characin pattern: an elongated, slender body suited to resting on or near the bottom rather than cruising the water column, with a build that favours short, precise movements over sustained open-water swimming.
As with many small, poorly studied Crenuchidae, detailed morphological description in accessible literature is limited, and this article does not go further into finnage or colour pattern specifics than the cached sources support — data is sparse beyond the confirmed maximum size and general body form.
Habitat
The species is native to coastal stream systems of northern South America, specifically in Suriname and French Guiana, within the Guiana ecoregion. FishBase describes its habitat as well-oxygenated, fast-moving creeks with clear water, though it notes the species can also adapt to swampy environments.
Aquarium-care sources describing the broader darter-characin group (Characidium and relatives) emphasise a strong current and heavy oxygenation — turnover on the order of ten to twenty times tank volume per hour — reflecting the flowing, well-aerated creek habitat these fish come from in the wild. Water is generally on the acidic side; hobby guidance for this group favours soft, black or sieved (peat-filtered) water conditions.
Feeding
Microcharacidium eleotrioides is a micro-predator rather than an indiscriminate omnivore. Aquarium-care guidance for darter characins describes them as carnivorous, taking dry, fresh and frozen foods, but food should be offered in amounts the fish can consume within two to three minutes, since these are small-mouthed, deliberate feeders rather than aggressive competitors at the surface.
Given the tiny adult size of this species, offering correspondingly small food items — micro pellets, finely crushed flake, or small live and frozen foods such as microworm, baby brine shrimp and cyclops — is consistent with its natural diet of small invertebrates picked from the substrate and current, rather than flake alone.
Mating
Specific courtship behaviour for Microcharacidium eleotrioides is not documented in the cached sources. In Crenuchidae more broadly, the sailfin characins (Crenuchus, Poecilocharax) are known cave or leaf spawners with brief male care of the clutch, while species in the related darter genus Characidium scatter eggs over the substrate without parental care. Data is too sparse for this specific species to state which pattern it follows with confidence, and this article does not guess.
Breeding
No breeding account for Microcharacidium eleotrioides was found in the cached sources, and given its tiny size, restricted range and rarity in the trade, it is not established as a regularly home-bred aquarium fish. Rather than assign a breeding mode by inference from relatives, this article notes plainly that data is sparse: some Crenuchidae are cave/leaf spawners with male care and others are substrate egg-scatterers, but which (if either) applies to this species is not confirmed here.
In the aquarium
This is a nano species in the truest sense, with an adult size of only about 1 in SL, and it should be kept and shopped for with that in mind — it is not a fish for a general community tank of larger tankmates. Hobby guidance for darter characins as a group recommends keeping them in groups of six or more in tanks of 10 US gal upward, with strong water movement and high oxygenation replicating the fast-flowing creeks they come from, rather than the still, warm water many nano fish tolerate.
Water should be kept acidic and soft, ideally filtered over peat or otherwise sieved to mimic blackwater conditions, at a moderate temperature around 72–77 °F. Nitrate should be kept low (guidance for the group cites under 25 mg/L), and these fish are described as sensitive to abrupt changes in water parameters and to medications generally, making them better suited to an experienced keeper's mature, stable tank than to a new setup. Tankmates should be limited to other small, peaceable species that will not outcompete them for food or bully them out of cover.
Conservation
Microcharacidium eleotrioides is assessed by the IUCN Red List as Least Concern, an assessment dated June 9, 2020. It is not a species of known conservation concern, though as with many small, range-restricted coastal-stream characins of the Guianas, ongoing pressures on freshwater habitat quality in the region are the kind of long-term watch item relevant to species with a naturally narrow distribution, even where the species itself is not currently threatened. It does not appear to be a significant part of the aquarium trade, and no wild-collection pressure specific to the aquarium hobby is documented in the cached sources.