Home / Fishes / Tetras, piranhas and pacus / Hemiodus unimaculatus
No federal permit to keepHemiodus unimaculatus
Hemiodus unimaculatus
Fishes › Tetras, piranhas and pacus (Characiformes) › Hemiodontidae (Hemiodus characins)
Care follows the hemiodus characins guide below.
Care at a glance
Species-specific aquarium care
- Tank: At least 66 in long and 27 in front to back for one adult (adults commonly reach about 13.0 in / 32.9 cm).
- Water: Freshwater.
- Temperature: 73–79°F (range recorded for this species).
- pH: 5.5–7.5.
- Hardness: 0–20 dGH.
- Layout: Uses both the bottom and open water: give open swimming space plus cover near the substrate.
- Diet: Eats a varied diet: mix quality prepared foods with frozen and live foods. FishBase classes it as a herbivore.
Calculated from this species' own FishBase record: tank size from its typical adult length and swimming zone, water values from its recorded ranges. The group guide below fills in general care.
Care guide
Species care notes
- Tank: a base of at least 47 × 18 in (roughly 64 US gallons); space needed is rated “large tank”.
- Water: a fairly wide range, from slightly acidic to slightly alkaline, 72–79 °F, pH 5.5–7.5, 1–20 dGH.
- Behaviour: temperament peaceful; a shoaling fish best kept in a group of six or more.
- Difficulty: Moderate (Seriously Fish rating).
Short excerpts from the species profile:
- Setup: “A biotope setup would consist, very simply, of a layer of a couple of inches of inert sand as substrate, with perhaps some smooth rocks, driftwood branches and twisted roots scattered around.”
- Diet: “Unfussy, and will accept most live, frozen and dried foods.”
- Tankmates: “Very peaceful but it can intimidate much smaller or more quiet species with its activity and size.”
- Breeding: “Not thought to have been bred in the hobby.”
From the Seriously Fish profile for this species, which has the full habitat, maintenance, diet, behaviour and breeding write-up 167
Hemiodus characins
Applies to most species in this group. Habitat: Hemiodus inhabit South American floodplain rivers and lakes, with H. quadrimaculatus also frequenting sandy rapids. Individual species can differ, so compare with the species numbers above and the linked sources.
Housing & setup
- For H. gracilis and H. unimaculatus, use at least 120 × 45 × 45 cm, approximately 243 litres or 64 US gallons, with clear swimming space.
- Fit a close-fitting lid: these nervous fish make powerful jumps.
- Provide approximately 2 inches of inert sand, scattered wood and planted or floating cover.
- Maintain strongly oxygenated water and stringent maintenance; provide especially good aeration for rapid-dwelling H. quadrimaculatus.
- For H. gracilis, source guidance gives pH 5.8–7.2 and 4–15 dGH; H. unimaculatus guidance gives pH 5.5–7.5 and 1–20 dGH.
Climate & lighting
| Water | 73.4–80.6°F |
Diet
- Prepared dried aquarium foods.
- Varied live or frozen foods.
- Plant-based foods or greenstuff as a regular dietary component.
Social life
- Keep in a conspecific shoal. Generally peaceful, but their size and activity can intimidate tiny or quiet companions; use similarly sized tankmates.
Health watch
- Hypoxia when aeration fails.
- Freshwater ich affecting skin and gills.
Understanding the animal
In the wild
In the wild
From this species’ own FishBase record (habitat, bottom type, water and breeding data), not its family. Good care copies these conditions.1
| Where it lives | Lives in freshwater, in rivers and streams. Uses both the bottom and open water. Occurs in fast-flowing rivers over rocky substrate. Abundant in the lower part of rivers. Feeds on detritus and periphyton. Mature at about 11 cm; breeds during the rainy season. |
| Water it lives in | Freshwater, 73–79 °F (23–26 °C), pH 5.5–7.5, hardness 20 dH. |
| Climate | Tropical climate. |
| Behavior to plan for | Breeding: nesters. |
What this means for care
- Keep water within its wild range, 73–79 °F.
Native climate (measured)
Native climate (measured)
| Warm-season highs | about 91°F (up to 94°F in the hottest sampled areas) |
| Cool-season lows | about 69°F (down to 61°F in the coldest sampled areas) |
| Average temperature | 77–81°F across sampled records |
| Relative humidity | 77–84% on average (driest month about 73%) |
| Rainfall | about 2175 mm a year |
Measured at 514 wild occurrence records sampled against WorldClim 2.1 climate data (1970–2000 averages). Air conditions in the range are ecological context, not enclosure settings: microhabitats such as burrows, leaf litter or basking rocks can be cooler, warmer or more humid.
From the sources
From the sources
Quoted from the matching sections of reputable sources for this exact species. Specialist databases are used first; Wikipedia (community-edited, CC BY-SA 4.0) only fills gaps. Other sources are short credited quotes.
| Where it lives | Home ranges Population dynamics Growth parameters Max. ages / sizes Length-weight rel. Length-frequencies Mass conversion Recruitment Abundance Life cycle Reproduction Maturity Maturity/Gills rel. — FishBase |
| Biology | Occurs in fast-flowing rivers over rocky substrate. Abundant in the lower part of rivers. Feeds on detritus and periphyton. — FishBase |
Data and studies
The numbers
The numbers
Each bar is what one source reports. Where several sources weigh in, the dashed gold band marks their average.
Typical adult weight
Average adult body mass in each database (FishBase gives the maximum recorded weight).
Adult length
Maximum recorded length. Measured as standard length (snout to the base of the tail, not counting the tail fin). Fish in home aquariums often stay smaller than wild records.
Water temperature
Range the species tolerates in nature; aim for the middle.
Water hardness
Minimum tank footprint
Seriously Fish recommends a tank base of at least 47 × 18 in (about 64 US gallons).
Sources for this sheet
- FishBase (Froese & Pauly, eds.), version 24.07 Peer-reviewed researchThe global scientific database on fishes, maintained by a consortium of research institutions; every value is referenced.
- Seriously Fish species profiles Experienced keeper or breederLong-running aquarium reference written and peer-checked by experienced fishkeepers and ichthyologists; each profile covers habitat, tank size, water, diet, behaviour and breeding.
- Seriously Fish: Hemiodus gracilis Experienced keeper or breederAdded in the 2026 source-checked care-guide review.
- Seriously Fish: Hemiodus unimaculatus Experienced keeper or breederAdded in the 2026 source-checked care-guide review.
- FishBase: Hemiodus quadrimaculatus Peer-reviewed researchAdded in the 2026 source-checked care-guide review.
- Merck Veterinary Manual: Common Environmental Hazards for Fish Zoo, university or veterinaryAdded in the 2026 source-checked care-guide review.
- Merck Veterinary Manual: Parasitic Diseases of Fish Zoo, university or veterinaryAdded in the 2026 source-checked care-guide review.