Kubotai rasboras can breed in a mature planted aquarium, but finding the eggs is often easier than raising the fry. Adults scatter eggs among plants and provide no parental care. The newly free-swimming fry need food small enough to swallow, and the spawning setup must protect eggs without sacrificing water quality.
For a controlled attempt, condition a healthy group, move a pair or small mixed-sex group into a prepared spawning container with fine-leaved plants or spawning mops, and remove the adults after spawning—or after roughly two to three days if you cannot see the eggs. Expect hatching at around three days after spawning and free swimming another three to four days later. These are approximate, temperature-dependent intervals, not a calendar that every batch will follow.
The priorities: mature, stable water; appropriately sized food for the adults; dense spawning material; protection from egg predation; and a dependable supply of microscopic first food. A particular pH, medication, piece of driftwood, or number of daily feedings cannot guarantee success.
Kubotai rasbora: quick breeding facts
| Question | Practical answer |
|---|---|
| Scientific name | Microdevario kubotai; formerly Microrasbora kubotai. |
| Other names | Kubotai rasbora, neon green rasbora, green neon rasbora. |
| Adult size | About 2 cm standard length (measured without the tail fin). |
| Permanent aquarium | At least a 60 × 30 cm footprint (roughly 54 L in a typical tank); keep a shoal of at least 8–10. |
| Published maintenance water range | 20–27°C (68–81°F), pH 6.0–7.0, general hardness 1–10 °dGH. These are husbandry ranges, not proven breeding thresholds. |
| Breeding method | Egg scatterer, usually among fine vegetation; no parental care. |
| Separate spawning setup | A mature, clean container with matching water and plenty of fine-leaved plants or spawning mops. A pair or small mixed group can be used temporarily. |
| Adult removal | Promptly after spawning is observed; otherwise, a two-to-three-day spawning window is a documented method. |
| Approximate hatching | Around 72 hours after spawning, depending on temperature. |
| Approximate first free swimming | About 3–4 days after hatching, or roughly 6–7 days after spawning. |
| First food | Very fine live food or suitable micro-sized prepared food. A species account recommends an initial food grade around 5–50 µm; actual prey suitability depends on the fry’s mouth size and the organism’s dimensions. |
The species-specific husbandry and breeding timings above follow the Microdevario kubotai account at Seriously Fish. Use the figures as starting guidance and let observations of your own fish take priority over a rigid schedule.
Where kubotai rasboras come from
The green fish sold as the kubotai rasbora is Microdevario kubotai, not a species of Boraras. Its original scientific name was Microrasbora kubotai; the accepted combination is recorded in Eschmeyer’s Catalog of Fishes.
The species was described from Thailand. Detailed aquarium species accounts also report populations in neighboring Myanmar and describe headwaters and small tributaries with well-oxygenated, clear to sometimes slightly turbid water. Substrates can include rock, gravel, sand, roots, leaf litter, and vegetation. That is a stream-and-tributary habitat, not evidence that every breeding tank must be acidic blackwater or have powerful current.
Aim to reproduce the useful features of that habitat: clean water, dependable oxygenation, places to shelter, and room for a shoal. Do not chase an exact imitation of one collection site at the expense of stability.
Kubotai rasbora care before breeding: tank size, group and water
Keep a proper shoal in the permanent aquarium
For everyday kubotai rasbora care, use an established aquarium with at least a 60 × 30 cm footprint, suitable swimming room, and a shoal of 8–10 or more, provided filtration and stocking allow it. Plants or other cover help create secure areas, while an open section gives the fish room to move. Keep the tank biologically mature; this is not a species to introduce into an uncycled aquarium.
A small spawning container is not the same thing as permanent housing. A pair or a few selected breeders may stay in a temporary container for a short, monitored spawning attempt. Do not place an entire shoal into a cramped 10 L tub for days and call that normal group housing. Adding more breeders can also increase the number of eggs eaten.
Females generally become deeper-bodied and often larger as they mature. Males tend to be more compact and intensely colored. A rounded abdomen can suggest a gravid female, but shape alone does not prove that she is carrying eggs.
Use these water parameters as a guide, not a magic formula
The published care range is 20–27°C (68–81°F), pH 6.0–7.0, and 1–10 °dGH. A temperature in the middle of that range and water similar to the adult tank are reasonable starting points for a breeding attempt. No reliable species-specific trial establishes that every successful spawn requires pH 6.0–6.5, GH 2–4 °dGH, or KH below 2 °dKH.
If your water is well outside the published care range, investigate its GH, KH, and pH before attempting breeding. Where softer water is needed, mix reverse-osmosis (RO) water with a suitable mineral source or conditioned tap water outside the aquarium, test the resulting mix, and acclimate fish gradually. Pure RO water is not a ready-made long-term fish habitat: it lacks buffering and minerals. Peat may alter chemistry unpredictably, so test rather than assuming it delivers a specific pH.
Do not confuse GH with TDS. GH measures calcium and magnesium hardness; a TDS meter estimates total dissolved substances and cannot tell you GH or KH. If hardness is expressed as ppm in a GH conversion, it typically means ppm as calcium carbonate—not ppm of all dissolved solids.
Maintain good oxygenation and consistent temperature. Avoid sudden cold-water changes, aggressive pH adjustment, and strong flow blasting eggs or fry. There is no confirmed kubotai-specific requirement for a powerhead, a rainstorm simulation, or a barometric-pressure drop to trigger spawning.
How to breed kubotai rasboras: the controlled setup
A densely planted display tank can occasionally produce surprise fry, but a separate spawning setup offers a clearer way to protect eggs and control feeding.
Step 1: Prepare the spawning container before moving fish
Choose a clean container that offers enough room for the specific number of breeders you intend to use. A modest container may be suitable for a brief pair-based attempt; larger groups need more space and closer attention to oxygen and water quality. There is no research-backed universal 10–15 L requirement for this species.
Use conditioned water that closely matches the established adult aquarium. Add plenty of Java moss (Taxiphyllum spp.), other fine-leaved plants, or fish-safe spawning mops. These create surfaces and hiding places for scattered eggs. A bare bottom beneath the plants or an appropriately sized, secure egg-protection mesh can make inspection easier; verify that adults cannot become trapped and that eggs can pass through if you use mesh.
Keep lighting subdued if the fish appear nervous. Provide gentle aeration or a protected, mature air-driven sponge filter if needed; avoid exposed intakes and high-velocity jets. A new sponge filter does not become biologically mature just because it is wet. Check ammonia and nitrite, particularly in small unfiltered containers, and use a cover that allows gas exchange and prevents jumping.
Do not add methylene blue automatically. A universal instruction such as “two or three drops” gives no usable concentration because products and water volumes differ. Studies in other ornamental fish show species-dependent benefits and risks, and a safe, effective protocol for kubotai eggs has not been established. Clean equipment, healthy broodstock, careful egg inspection, and water quality come first. See the review of ornamental-fish treatments.
Step 2: Condition the adults without fouling the tank
Feed healthy adults a varied diet of appropriately sized foods: a quality fine dry food plus small live or frozen invertebrates such as suitable Daphnia and Artemia. Offer portions the fish can finish; remove uneaten food. A period of enhanced feeding before the attempt is practical breeder husbandry, not a proven two-to-four-week biological requirement.
Observe the fish. Rounded adult females and more intense male coloration can help you select breeders, but neither sign guarantees imminent spawning. Do not intentionally make the fish overweight, rely exclusively on one live food, or overload the water with repeated large meals.
Step 3: Introduce a pair or small group, then protect the eggs
A single well-conditioned pair is one documented controlled-breeding approach. A small group containing males and females is another; it increases the opportunity for courtship but also increases the risk of egg predation. Keep the main shoal in its proper aquarium and transfer selected adults gently into the prepared container.
Leave them relatively undisturbed, monitoring respiration, behavior, and water quality. Watch among the moss or mops for tiny eggs and for courtship, but do not rely on a particular “golden” color or side-by-side swim as proof that eggs were laid. Spawning can occur repeatedly in healthy fish, but daily spawning and a guaranteed egg count are not established outcomes.
If you see eggs or spawning, remove the adults promptly with minimal disturbance. If eggs are hard to detect, removing the adults after roughly two to three days is the approach described by Seriously Fish. Waiting a fixed 72 hours after seeing spawning may leave eggs exposed to predation unnecessarily.
Kubotai rasbora eggs: what happens after spawning?
Kubotai are egg scatterers: eggs are deposited among plants and receive no parental care. Adults may consume eggs or fry, which is why protected spawning media and timely adult removal matter.
Count the incubation clock from the time the eggs were laid. Hatching is reported at approximately 72 hours, with temperature affecting development. Newly hatched larvae are not immediately free-swimming. The transition to free swimming is reported approximately three to four additional days after hatching.
If an egg turns opaque or develops visible fuzzy growth, it may be infertile or compromised; an isolated white spot is not sufficient to diagnose the cause. Where eggs can be accessed safely, remove visibly fungused eggs using clean tools without tearing apart the entire nursery. Do not raise the temperature, pour in an unmeasured antifungal, or expose larvae to strong flow as a reflex.
A timeline that keeps the stages separate
| Time relative to spawning | What to watch for | Your job |
|---|---|---|
| Day 0 | Eggs may be deposited in plants or mops. | Remove adults when spawning is identified; protect eggs. |
| Around day 3 | Hatching may begin. | Maintain stable, clean water; avoid disturbing larvae. |
| Around days 6–7 | Some fry may begin free swimming, approximately 3–4 days after hatching. | Offer appropriately sized food when the fry actually begin feeding. |
| Following days and weeks | Mouth size, swimming ability, and appetite increase gradually. | Increase prey size only after observing successful swallowing. |
This is an approximate species-account timeline, not a guarantee that every batch hatches or feeds on those exact days.
Feeding kubotai rasbora fry: start smaller than you think
The most consequential feeding mistake is offering food that looks fine to you but is too large for a newly free-swimming fish. The species account recommends an initial food grade of roughly 5–50 micrometers (µm), followed by larger prey such as Artemia nauplii or microworms once the fry can accept them. That range describes a suggested first-food size, not the dimensions of every organism sold as “infusoria.”
Before free swimming: have two food options ready
Prepare a dependable small-organism culture before you expect the eggs to hatch. “Infusoria” is a loose hobby term for mixed microscopic organisms, not a single species with a consistent size or nutritional profile. A culture dominated by larger Paramecium may be unsuitable for the smallest fry: for example, Paramecium caudatum is around 250 µm long in a published laboratory study—well above the cited 5–50 µm first-food grade.
For a practical backup, use a clean starter culture of suitably small microorganisms and/or a commercial fry food with a verified particle size in the appropriate range. Assess actual feeding under magnification if possible. A jar that turns cloudy after adding vegetable waste is not proof that it contains the right prey, and adding murky or foul-smelling culture water to a fry tank risks contaminating it. Harvest the organisms with as little culture medium as possible.
At first feeding: use small portions and watch the fish
Begin feeding when fry are visibly free-swimming and attempting to eat. Introduce small amounts distributed where the fry are feeding; observe whether they capture and swallow the prey and whether food remains in the water. Several restrained feedings can work better than a single large dump, but no universal “three to five feedings” schedule has been validated for kubotai. Adjust to prey density, the number of fry, tank cleanliness, and observed feeding.
Green water may support a community of suitable microorganisms, but algae alone should not be assumed to meet the fry’s dietary needs. Likewise, do not assume all vinegar eels or every Paramecium species are small enough just because they are sold as fry food.
When can kubotai fry eat microworms or baby brine shrimp?
Move up only when the fry can swallow larger items. Microworms and newly hatched brine shrimp (Artemia nauplii) are established later-stage options, not automatic first foods for this species. There is no reliable universal rule that every kubotai fry is ready on day 14 or day 21. If a test portion is ignored, too large to swallow, or falls out of reach, continue with smaller foods and try again as the fry grow.
For culture preparation and safe harvesting, see NanoFishNest’s microworm culture for fry guide. Remember that microworms tend to sink: whether they are useful also depends on where your fry are feeding. As the young grow, expand the diet rather than relying indefinitely on a single live food.
Does biofilm on driftwood keep fry alive?
Mature moss, leaves, and established surfaces can host microscopic organisms that supplement the food available in a nursery. They are useful options if they come from a healthy, pest-free aquarium. They are not a substitute for an adequate, identifiable first-food culture.
There is no verified species-specific evidence that kubotai fry must have aged driftwood or Indian almond leaves to survive. Do not transfer questionable material from another tank solely to introduce biofilm, and do not add enough decomposing leaf matter to compromise a tiny nursery’s water quality.
Fry tank water changes: protect the fry and the water
The original advice to avoid water changes for two weeks was too absolute. Tiny fry are vulnerable to suction and abrupt temperature or chemistry shifts, but waste and uneaten food can also damage them. Topping off evaporation is not a substitute for removing waste.
Test ammonia and nitrite regularly, with both kept at undetectable levels using a suitable kit; monitor temperature and watch for leftover food. When waste accumulates or water quality requires it, perform a small, carefully controlled partial water change rather than waiting for a particular calendar day. Use a narrow airline siphon with its intake protected by fine mesh or another fry-safe guard, check the removed water for fry, and refill slowly with dechlorinated replacement water matched closely for temperature and relevant chemistry. Choose the change volume according to tank size and test results—not a universal number of liters.
For evaporation-only top-ups, use suitably clean RO or distilled water when appropriate: evaporated water leaves its minerals behind. For actual water changes, use the tested, appropriately mineralized water mix needed for the fish. Do not use untreated tap water or pure, unbuffered RO as a blanket replacement-water rule. In an emergency with detectable toxic nitrogen compounds, prioritize prompt correction of water quality over a rigid “no changes” policy.
Troubleshooting: why aren’t my kubotai rasboras breeding?
| What you see | Plausible explanations | What to do |
|---|---|---|
| No apparent spawning | Fish not mature or conditioned, stressed by transfer, unsuitable chemistry, or eggs hidden in moss. | Check species ID, condition, stability and water tests. Allow time; inspect spawning mops carefully rather than repeatedly changing pH. |
| Eggs disappear | Adults may eat them; eggs can also be difficult to see. | Use dense mops or moss and remove adults as soon as spawning is confirmed. |
| Eggs become opaque or fuzzy | Some may be infertile or affected by microorganisms; the appearance alone does not establish the cause. | Check cleanliness and water quality; remove clearly affected eggs where safely accessible. Avoid unmeasured medication. |
| Fry hatch but don’t begin feeding | Larvae may still be absorbing yolk; later, available prey may be the wrong size or density. | Distinguish hatching from free swimming. Supply verified small first food when feeding starts; observe ingestion. |
| Fry disappear after feeding begins | Insufficient appropriate food, water-quality deterioration, suction, predation, or other causes. | Test water, review food size and feeding observations, protect intakes, and avoid assuming “missing biofilm” is the answer. |
| Losses follow a water change | Temperature or chemistry shift, chlorine/chloramine exposure, or physical injury from siphoning. | Test source and tank water, condition replacements, change water slowly, and inspect siphoned water. |
Kubotai rasbora breeding FAQs
Do kubotai rasboras breed easily?
They have been bred successfully in aquaria and may produce occasional fry in a mature planted tank. Raising larger numbers requires protecting eggs and providing sufficiently small first food. A successful spawn does not guarantee a successful batch of juveniles.
What water parameters do kubotai rasboras need for breeding?
Published general-care guidance is 20–27°C, pH 6.0–7.0 and 1–10 °dGH. Stable, clean water within the species’ requirements is a sensible starting point, but an exact breeding-only pH, GH or KH threshold has not been established for all populations and setups.
What size tank does a kubotai rasbora need?
For permanent care, use at least a 60 × 30 cm footprint, around 54 L in a typical aquarium, with space for a group of at least 8–10. A smaller container may be used briefly for carefully monitored spawning by a pair or small group; it is not permanent housing.
How can I tell male and female kubotai rasboras apart?
Mature females are usually larger and deeper-bodied. Males are generally slimmer and more intensely colored. These differences are subtle in young or poorly conditioned fish, so avoid treating one rounded belly as definitive proof of sex or imminent spawning.
How long do kubotai rasbora eggs take to hatch?
Approximately 72 hours after spawning is the reported figure, with temperature affecting incubation. Free swimming usually follows about three to four days after hatching, so do not count both events as occurring on the same day.
What do newly hatched kubotai rasbora fry eat?
Before they are free-swimming, larvae draw on their yolk reserves. Once they begin feeding, offer organisms or verified prepared particles small enough to swallow; the species account suggests a 5–50 µm initial food grade. Larger Paramecium, vinegar eels, microworms, and baby brine shrimp are not universally suitable at first feeding.
Can kubotai rasboras breed in a community tank?
They may spawn in a mature planted aquarium, but adults and other tank residents can eat eggs or fry. A separate, carefully maintained spawning and rearing setup makes protection and feeding easier if you want to raise more offspring.
Can I keep kubotai rasboras with shrimp?
Peaceful adult fish and shrimp may coexist in a suitably sized, compatible aquarium, but tiny shrimp young and fish eggs may be vulnerable to predation. Do not assume a shrimp community is an egg-safe breeding setup; separate breeders when maximizing fry survival is the goal.
The practical takeaway
Start with healthy kubotai in a mature, appropriately sized shoal aquarium. Prepare a separate moss- or mop-filled spawning container using stable, matching water, move a selected pair or small group for a short attempt, and protect the eggs from adults. Have genuinely small first food ready before free swimming, then introduce microworms and baby brine shrimp as mouth size allows. Keep water clean through testing and careful maintenance, not by banning water changes.
The species has been bred in aquaria. What remains uncertain is any single “perfect” pH, exact egg yield, universal first-food day, or guaranteed survival percentage. Keeping those uncertainties visible makes this a more useful breeding guide—not a less useful one. For a broader breeding-room overview, read the nano fish breeding guide.
References and further reading
- Seriously Fish — Microdevario kubotai: habitat, aquarium requirements and reproduction. Species-focused aquarium account; source for the husbandry ranges and approximate spawning, hatching, free-swimming and first-food guidance.
- Eschmeyer’s Catalog of Fishes — M. kubotai. Taxonomic authority for the accepted scientific name and original combination.
- FishBase — Microdevario kubotai. Independent taxonomic and size context; its brief distribution summary lists Thailand, whereas the detailed Seriously Fish account also discusses Myanmar records.
- Larcombe and colleagues — review of ornamental fish disease treatments. Explains why methylene-blue efficacy and safety cannot be assumed to transfer between fish species.
- Scientific Reports — Paramecium caudatum dimensions. Illustrates why naming “paramecium” alone does not demonstrate that a culture meets a 5–50 µm first-food specification.
Evidence note: The breeding protocol combines documented species-account observations with cautious, practical aquarium husbandry. Controlled, species-specific experiments are limited. No survival rate or exact breeding outcome is promised.