Least pencilfish
Nannostomus minimus
Least pencilfish exhibits a slender, elongated body, with striking vertical black stripes and a translucent, iridescent sheen.
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About the Least pencilfish
Think of these as tiny floating dashes of color that cruise just under the surface in a loose group. They stay under an inch, show a crisp dark stripe with little red flecks, and really come into their own in soft, tea-colored water with plants and leaves. Super chill, but they do best in a decent-sized group so they feel secure.
Quick Facts
Size
2.3 cm SL
Temperament
Peaceful
Difficulty
Intermediate
Min Tank Size
15 gallons
Lifespan
3 years
Origin
South America (Guyana)
Diet
Carnivore - micro live and frozen foods; accepts fine flakes and micro-pellets
Water Parameters
22-26°C
4-6.5
1-8 dGH
Need a heater for this species?
This species needs 22-26°C in a 15 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Start with a group of 10-12 in a 15 gallon long; they are tiny but need elbow room and cover to chill out.
- Run a sponge filter with gentle flow, tight lid, dim light, floating plants, and some leaf litter.
- They like soft, acidic, tea-colored water: pH 4.5-6.5, GH 1-5, 75-82 F.
- Keep nitrates under 10 ppm and do small weekly changes, not big swings.
- They are micro-predators with pinhole mouths, so feed small portions 2-3x daily.
- Baby brine shrimp, cyclops, daphnia, and fine powders or crushed micro-pellets work great.
- Pick calm nano buddies only: other small pencils, ember or chili rasboras, pygmy corys, and otos.
- Skip guppies, danios, barbs, bettas, and anything that hogs food or nips.
- They hang near the surface and hate strong current; baffle the outflow so the top stays quiet.
- Dark substrate and tannins bring out the stripes and confidence.
- For spawning, use a dim 5-10 gallon with very soft acidic water, a spawning mop or fine plants, and tons of microfoods for conditioning.
- They scatter tiny eggs and will eat them, so pull the adults; start fry on infusoria, then baby brine after 3-4 days.
- Wild-caught fish often carry parasites, so quarantine and deworm with levamisole or praziquantel once they are eating.
- They are sensitive to copper and aggressive dosing; add extra aeration if you medicate.
- They jump when spooked, so seal every lid gap and keep the water line an inch below the rim.
- Go easy on sudden bright lights or banging near the tank.
Compatibility
Good Tankmates
- Tiny, chill schoolers like chili rasboras, ember tetras, or green neons
- Other small pencilfish that mind their manners, like Nannostomus marginatus or N. eques
- Peaceful bottom crews like pygmy corydoras or dwarf corys
- Otocinclus groups that keep to the glass and leaves
- Shrimp and snails for cleanup - adults are fine, expect them to munch some shrimplets
- Mellow bottom noodles like kuhli loaches
Avoid
- Anything nippy or hyper, like tiger barbs, serpae tetras, or zebra danios
- Bettas or gouramis that guard the surface and outcompete at feeding time
- Bigger cichlids and angels that see tiny pencils as snacks
- Surface-hunting killifish that strike fast
Where they come from
Least pencilfish are tiny blackwater fish from northern South America, showing up in slow forest creeks and flooded swamps in places like Guyana and Suriname. Picture tea-colored water, leaf litter, overhanging branches, and almost no current. They spend their days picking at micro-life in the safe shade of plants and twigs.
They come from very soft, acidic water stained by leaves. You do not have to run full blackwater at home, but they relax a lot more with tannins and dim light.
Setting up their tank
Go for a mature, low-flow setup with a tight lid. A 10-gallon works for a small group, but they look better and act bolder in a 15-20 long. Use a sponge filter or a gentle canister with a spray bar aimed at the glass. Floating plants, fine-leaved stems, and a handful of catappa or oak leaves help a ton. Keep the lighting soft.
- Group size: 10+ is best so they stop hiding and show color
- Temperature: 75-82 F (24-28 C)
- pH: 4.5-6.8 preferred; they can adapt up to around 7.0 if stable
- Hardness: low - under ~6 dGH, low KH
- Flow: gentle; they hate being blasted
- Water quality: ammonia and nitrite 0, nitrate kept low
Let the tank run and settle for a few weeks before adding them. Add botanicals gradually, and put a sponge pre-filter on any intake so the little ones do not get sucked in.
They jump. Use a snug lid and cover any gaps, especially around filter pipes and cables.
What to feed them
They have tiny mouths and act like micro-predators. If food is too big or sinks fast, they just stare at it. Offer small, moving foods often and use dry foods as a supplement, not the whole menu.
- Live: freshly hatched baby brine shrimp, moina/daphnia, grindal/microworms, vinegar eels for fry
- Frozen: cyclops, baby brine, daphnia (thawed and swirled so it stays suspended)
- Dry: fine powders and true nano pellets, crushed high-quality flakes
Two to three small feedings a day beats one big one. Pre-soak dry foods so they hang in the water column. If you can culture bbs or microworms, these fish reward you with better color and fuller bellies.
How they behave and who they get along with
Peaceful, shy, and very group-focused. They hang in the mid to upper water and hover in little squads, then dart for tiny snacks. Males spar a bit with flared fins but it is harmless if the group is big enough and there are plenty of plants.
- Good tankmates: other gentle nanos like ember tetras, small rasboras, chili rasboras, dwarf corys (pygmaeus/habrosus), otocinclus, small peaceful Apistogramma kept mellow, kuhli loaches in bigger setups
- Shrimp: adults are fine, but shrimplets are snacks
- Avoid: fin-nippers, anything boisterous, and fish that need strong current
They get outcompeted easily. If you mix them with faster feeders, they lose. Feed in multiple spots and float food so it stays where pencilfish like to eat.
Breeding tips
They scatter eggs among fine plants and do not guard them. A dedicated breeding box or nano tank makes it doable. Males are a bit slimmer and more vivid; females carry a fuller belly when ready.
- Setup: 5-10 gallon with sponge filter, dim light, clumps of moss/fine plants, a layer of leaf litter
- Water: very soft and acidic (pH 5-6.5), warm 78-80 F
- Conditioning: several days of live and frozen foods for the group or a 1:1-2 male to female pair
- Trigger: small cool water change and lights on low in the morning
- After spawning: pull adults the same day; they eat eggs
- Eggs/Fry: eggs hatch in ~24-36 hours, free-swimming a few days later; start with infusoria/green water, then vinegar eels or microworms, then baby brine by the end of week 1-2
Keep the tank dim and cover the filter intake with fine sponge. A couple of catappa leaves help with tannins and biofilm, which is free first food for fry.
Common problems to watch for
- New fish arrive skinny and get pushed off food - fix with live foods and calm tankmates
- Jumping during water changes or startled by bright lights
- Ich after temperature swings; they are small and do not tolerate sudden changes
- Stress bars and fin clamping under harsh light or strong flow
- Ammonia and nitrite sensitivity - they need a mature, stable tank
- Sucked into filter inlets without a pre-filter sponge
Quarantine new arrivals. Wild-caught fish sometimes carry worms; a round of levamisole or praziquantel during quarantine has saved me headaches later.
If they hide and refuse food, dial back the light, add floating plants, split feedings into tiny portions, and check your flow. Steady, small water changes beat big swings for these guys.
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