New Guinea Red Rainbowfish (Glossolepis incisa)

Understand the fish before choosing it

The New Guinea Red Rainbowfish (Glossolepis incisa) is a large, active freshwater rainbowfish from the family Melanotaeniidae. FishBase records it as endemic to Lake Sentani in Indonesian New Guinea, with a maximum of 12 cm standard length, and lists the wild population as Endangered. It suits a keeper prepared to maintain a proper shoal, a long open swimming lane and mineral-bearing water. Its rapid movement and relatively large adult body make it poorly suited to a cramped aquarium or a community built around very small, timid fish.

FishBase lists the scientific name as Glossolepis incisa. The older aquarium spelling Glossolepis incisus also appears in established husbandry writing, but both names refer here to the New Guinea Red Rainbowfish.

Plan with two adult-size references

FishBase records a maximum of 12 cm standard length for male or unsexed fish. Standard length excludes the tail. Seriously Fish uses 15 cm standard length as its aquarium-planning reference, so allow for a substantial, deep-bodied adult rather than treating the dispatch size as the finished size. The 15 cm figure is a planning reference, not a guaranteed final measurement for every individual.

Mature males develop the characteristic red colour and a higher back as they grow. Colour should be treated as adult biology rather than a promise about the appearance or sex of an individual animal selected for dispatch.

Choose the dispatch grade and plan for adults

One purchase supplies one animal in the selected size. The live size selector shows 4.5–5.5 cm, 6–7 cm and 7–8 cm. Select the size there, then set the quantity to match the number of animals you intend to receive.

The size you select is the size supplied, not the adult size. For example, selecting six animals at 4.5–5.5 cm means planning immediately for six growing rainbowfish, not planning the aquarium around six fish that will remain close to 5 cm. Use the adult group, swimming footprint and filtration load for every setup decision.

Read the offer separately from adult appearance

Sex is guaranteed only when the selected option explicitly states male or female. This New Guinea Red Rainbowfish listing has no sex-guaranteed option.

Do not calculate a precise male-to-female ratio from size or colour. Mature male colour and body shape are useful biological characteristics, but they do not change the terms of this offer.

The gallery images are representative, based on reference photographs of this species, so colours can look slightly different from the live animal.

Choose the dispatch size only after confirming that the aquarium can support the full group. A larger starting size changes the fish received, but it does not reduce the need for a shoal, open water, mature biological filtration or compatible companions.

Give the shoal a long, open swimming lane

Seriously Fish recommends an aquarium at least 120 × 30 × 30 cm for this active swimmer, with a volume of roughly 108 litres. The 120 × 30 cm base is the decisive measurement because it establishes the length and turning width of the swimming lane.

OATA’s rainbowfish guidance uses at least 120 litres for larger rainbowfish over 8 cm. A practical aquarium should therefore provide the species footprint and at least 120 litres, with additional space when the group is larger or the aquarium contains other complete social groups.

Divide the aquarium into cover and swimming space

• Back and sides: use areas of dense live vegetation to give the group visual cover.
• Centre and front: preserve open water so the fish can move rapidly without threading through continuous planting.
• Flow: keep water movement gentle rather than turning the swimming lane into a strong current.
• Substrate: aquarium gravel or sand can be used beneath the planted and open areas.

This arrangement lets a skittish group move into cover while preserving the open space required by an active rainbowfish. Filling every part of the aquarium with dense décor would remove the main swimming area even if the nominal volume remained unchanged.

Match equipment to the chosen volume

For the roughly 108-litre example, Seriously Fish recommends filtration turning over four to five times the aquarium volume each hour, equivalent to a filter rated around 430–540 litres per hour. It also gives a heater reference of roughly 75–100 W for holding 22–25°C when the room is around 20°C.

Those equipment figures belong to the 108-litre worked example. Choose equipment for the actual aquarium volume and room conditions when using a larger tank. Preserve gentle movement in the display while ensuring that water reaches the biological filter reliably.

Hold the species aquarium range steady

Use 22–25°C, pH 7.0–8.0 and 10–20 dGH as the routine aquarium range. These species husbandry values from Seriously Fish are the appropriate targets for aquarium planning and are narrower than broad rainbowfish ranges.

This rainbowfish does not do well in soft, acidic conditions. Test the aquarium and source water before selecting it, paying particular attention to hardness as well as pH. A pH reading alone cannot show whether the water contains the mineral content represented by 10–20 dGH.

Use the range as a boundary, not a target to chase

An aquarium reading of 24°C, pH 7.5 and 12 dGH falls inside all three ranges. There is no need to force those values towards a single midpoint. Instead, keep the aquarium inside the stated boundaries and avoid abrupt movement between readings.

If a test gives 6.8 pH or 8 dGH, the water is outside the species aquarium range even though the difference may look small. Resolve the preparation method before introducing the shoal rather than relying on repeated sudden adjustments around the fish.

Our water chemistry guide explains temperature, pH, general hardness, ammonia, nitrite and nitrate. Enter measured values into the water matcher to compare the aquarium against the stated range.

FishBase also records warmer environmental temperatures and slightly different chemistry from the natural habitat. Treat those as natural-history values, while using the 22–25°C species aquarium range for routine care.

Build maintenance around the biological filter

A mature biological filter is essential before the group is introduced. The RSPCA’s aquarium welfare guidance explains that fish release toxic ammonia, which filter bacteria convert first to toxic nitrite and then to safer nitrate. A filter must therefore be established, working and maintained rather than treated as simple water circulation.

If this is your first setup, follow our guide to preparing and maturing a tropical aquarium before arranging livestock. A newly filled tank is not mature merely because the heater and filter are running.

Use a repeatable weekly routine

For routine water quality, OATA’s group guidance sets ammonia and nitrite at zero, nitrate at no more than 20 mg per litre above the normal tap-water level, and recommends testing at least weekly. It also strongly recommends a 25% partial water change at least once every week.

1. Test ammonia, nitrite and nitrate before maintenance so the result reflects conditions since the previous change.
2. During that weekly task, siphon solid waste while removing 25% of the aquarium water.
3. Treat replacement tap water with conditioner to remove chlorine or chloramine.
4. Check the filter for blockages and confirm that water movement remains reliable.
5. Record the results so a developing trend is visible before fish behaviour changes.

In a nominal 120-litre aquarium, 25% is 30 litres. Measure the removed volume rather than estimating from a small change in water level.

Protect the filter bacteria during cleaning

Do not rinse biological media under the tap because chlorine or chloramine may kill its established bacteria. When cleaning is needed, rinse the media lightly in aquarium water removed during the partial water change. The aim is to clear obstructing debris without stripping away the bacterial population responsible for processing waste.

Increase livestock numbers slowly. More fish and more food increase the waste load, and the filter’s bacterial population needs time to respond.

Keep a complete social group

Keep New Guinea Red Rainbowfish in a shoal of at least 6–8 fish, preferably more. Seriously Fish describes the species as skittish and reports that it does much better in a proper shoal. OATA likewise explains that rainbowfish are social and will often become stressed when kept alone or in a group that is too small.

A pair or trio does not provide the recommended social setting. If the aquarium cannot house at least six adults with the full swimming footprint, filtration and water quality required by the group, choose a smaller species rather than reducing the number of New Guinea Red Rainbowfish.

Increase group size only with space to match

Mature males may display stronger red colour when kept with their own kind. They also develop a higher back than females as they grow. These characteristics can emerge with maturity, but they should not be used to assign the sex of an animal from this offer.

Watch how the shoal uses the aquarium. Healthy social activity should leave every fish able to move through open water and reach its normal feeding opportunity. A fish that begins hiding more than usual, changes temperament or separates from its established pattern needs closer observation and an immediate water-quality check.

More fish can create a more visible shoal, but group size cannot be increased independently of aquarium capacity. A very large group requires a suitably larger aquarium, even when the starting fish are from a smaller dispatch size.

Feed once daily with varied foods

New Guinea Red Rainbowfish accepts most dried, frozen and live foods. Use a good-quality flake, granule or other suitable dried food as the regular base, then include appropriately sized frozen or live items for variety. Seriously Fish notes that regular frozen and live feeding can support the best colouration.

OATA’s rainbowfish feeding guidance recommends one feed each day, limited to what the fish can eat within a few minutes. That schedule gives a clear portion test and reduces the risk of uneaten food decomposing into toxic waste.

Build a simple feeding rotation

Choose food pieces the fish can take cleanly. The listed examples are options for variety rather than a requirement to provide every item in one meal.

Let clearance time set the portion

1. Offer the daily portion and observe the group for a few minutes.
2. If food remains after that period, remove what you can and reduce the next portion.
3. If one individual repeatedly shows reduced interest while the rest feed normally, check its behaviour and condition rather than adding more food for the whole aquarium.
4. Use water-test results alongside feeding observations because excess food increases the waste processed by the filter.

Rapid feeding enthusiasm is normal for an active fish, but it is not a reason to continue adding food without limit. Consistent small portions protect water quality more effectively than occasional heavy feeds.

Prepare the aquarium before introduction

Complete the aquarium, establish the biological filter and confirm the species water range before the fish arrives. Test for 0 mg per litre ammonia, 0 mg per litre nitrite, 22–25°C, pH 7.0–8.0 and 10–20 dGH. Check that the group will enter an aquarium with the required swimming footprint and planted cover already in place.

Read the current delivery and Live Arrival Guarantee policy when planning receipt. It contains the delivery and claim details that apply to an order.

Follow the arrival steps in order

1. Be present to receive the parcel because live fish cannot be left unattended.
2. Start filming before the sealed parcel is opened, then record one continuous take showing the outer box, the parcel being opened and the fish still inside the sealed bag.
3. Open and inspect the parcel on the day it arrives.
4. Float the bag for 20–30 minutes before releasing the fish.
5. Never pour the bag water into the aquarium.

Use a first-day readiness check

• Filter: confirm continuous operation and no blockage.
• Water: record temperature, pH, hardness, ammonia and nitrite.
• Space: preserve the open swimming lane rather than adding last-minute décor.
• Stocking: add fish gradually instead of increasing the aquarium population too quickly.
• Observation: watch swimming, breathing, colour, condition and interest in food.

Acclimation begins with matching the animal to a suitable mature aquarium, not with trying to correct an unsuitable tank during introduction.

Use the first week to establish a baseline

Observe the group at the same times each day. Note how rapidly they settle into open-water swimming, whether every fish joins the shoal and whether each animal shows interest at the daily feed. Clear eyes, undamaged fins, intact scales and appropriate swimming provide useful reference points for later comparisons.

Test water during the first week rather than waiting for the next routine date if the aquarium population has increased. OATA warns that stocking too quickly can exceed the bacterial population’s ability to process extra waste, leading to unhealthy ammonia or nitrite.

Keep the introduction measured: stable water, a working filter, a complete group and close observation give the fish a stronger start than repeated changes to food, décor or chemistry.

Respond early to health changes

Learn the group’s normal pattern so that a change becomes obvious. Individual fish vary, but a marked departure in swimming, breathing, colour, temperament, appearance, body condition or feeding can be an early sign that attention is needed.

Start with measurable conditions

When behaviour or appearance changes, test ammonia and nitrite first and compare temperature, pH and hardness with the normal aquarium readings. Confirm that the filter is running and unobstructed. These checks can identify an environmental problem that affects the whole group.

Do not respond to a single symptom by adding food or repeatedly changing chemistry. Record what changed, when it began and whether one fish or the whole shoal is affected. If you are concerned about livestock health, contact us after testing the water so the observed signs and measurements can be considered together.

Review the first month

At the end of the first month, compare the group with the baseline from its first week. Look for steady participation in the shoal, consistent daily feeding and stable body condition. Review the recorded water tests at the same time; a gradual rise in nitrate or recurring trace of ammonia points to a husbandry issue even before dramatic behaviour appears.

Screen companions by size and pace

New Guinea Red Rainbowfish is described as very peaceful, but peaceful does not mean suitable for every community. Its rapid movement and relatively large adult size may disturb smaller or slow-moving fish even without predatory or aggressive behaviour.

Seriously Fish names similarly sized rainbowfish, characins, danios, barbs and catfish such as Corydoras as possible companions. Treat those as conditional examples, not automatic matches: each exact species must share the aquarium range and have enough room for its own adult size and social group.

Apply four screens before combining fish

Avoid very small fish. OATA advises against mixing larger rainbowfish with very small species because of the risk created by the size difference. Use extra caution with timid or slow-moving fish, which may be unsettled by constant fast movement or lose opportunities during community activity.

A similarly sized companion still fails the screen if it needs soft, acidic water or a temperature outside 22–25°C. Conversely, matching water does not rescue a combination in which a timid fish cannot settle around the rainbowfish shoal.

Browse the rainbowfish collection to compare identities and adult requirements, then assess each exact species rather than treating the family as interchangeable. If a proposed combination falls outside the examples here, contact us for a stocking and compatibility assessment with the aquarium dimensions, water readings and full livestock plan.

Test the whole stocking plan

Compatibility is a whole-aquarium decision. A companion may match the water and temperament yet still make the plan unsuitable if the aquarium cannot hold both species at adult size in their proper social groups.

Work through the plan in order

1. Reserve the rainbowfish group: allow for at least 6–8 New Guinea Red Rainbowfish, preferably more.
2. Check the footprint: start with a base of at least 120 × 30 cm and retain an open swimming lane.
3. Check the water range: every animal must suit 22–25°C, pH 7.0–8.0 and 10–20 dGH.
4. Compare movement: remove very small fish from the plan and examine whether timid or slow-moving species can cope with rapid activity.
5. Count complete groups: include every companion’s full social requirement rather than counting one example of each species.
6. Assess filtration: add livestock slowly so the biological filter can respond to increased waste and feeding.

Use a worked screening example

An aquarium measuring 120 × 30 cm at the base, running at 24°C, pH 7.5 and 12 dGH passes the basic footprint and water screens. Those measurements do not automatically establish capacity for six to eight rainbowfish plus another shoal and a catfish group. The total adult groups, open swimming area and filter load must still be assessed.

A companion described as peaceful but very small fails the size screen. A similarly sized fish that needs soft, acidic water fails the chemistry screen. A robust fish within the same water range may pass the initial screens, but it still needs sufficient adult space and an appropriate social group.

Species not named in the examples may still be suitable when all of these conditions align. Equally, a named characin, danio, barb, rainbowfish or catfish may be unsuitable when its exact water, size, activity or group needs conflict with the setup.

Answer key buying and care questions

How big does a New Guinea Red Rainbowfish get?

FishBase records a maximum of 12 cm standard length. Seriously Fish uses 15 cm standard length as an aquarium-planning reference, so allow space for a substantial, deep-bodied adult.

What size aquarium does a New Guinea Red Rainbowfish need?

Use a mature filtered aquarium with a base of at least 120 × 30 cm. Seriously Fish gives roughly 108 litres for a 120 × 30 × 30 cm aquarium, while OATA recommends at least 120 litres for larger rainbowfish over 8 cm.

How many New Guinea Red Rainbowfish should I keep together?

Keep at least 6–8 fish, preferably more. Seriously Fish describes this species as skittish and says it does much better in a proper shoal.

Does a New Guinea Red Rainbowfish suit a beginner?

It suits a keeper who can provide 6–8 fish, a 120 × 30 cm-base mature aquarium, stable mineral-bearing water and weekly testing and maintenance. It is a poor choice where those group, space and water requirements cannot be met.

What does a New Guinea Red Rainbowfish eat?

Feed good-quality flake or granules once daily, offering only what the fish eat within a few minutes. Add suitable frozen or live foods such as daphnia, small bloodworm, tubifex or brine shrimp for variety.

What temperature and pH does it need?

Keep it at 22–25°C and pH 7.0–8.0, with general hardness of 10–20 dGH, following Seriously Fish species guidance. Keep ammonia and nitrite at 0 mg per litre.

Is this fish aggressive or a fin nipper?

Seriously Fish describes it as very peaceful, but its rapid movement and relatively large size may disturb smaller or slow-moving fish. Screen companions for adult size, activity, water requirements and space.

Which animals can live with it?

Consider similarly sized rainbowfish, characins, danios, barbs and catfish such as Corydoras when their exact water, group and space needs match. Avoid very small fish and use caution with timid or slow-moving companions.

How does it arrive alive in the UK?

Be present to receive the parcel. Film every delivery continuously from the sealed parcel as it is opened, keeping the fish visible inside the sealed bag; then float the bag for 20–30 minutes before release and never pour the bag water into the aquarium.

Check the care bibliography

• FishBase: accepted identity, family, Lake Sentani distribution, environmental values, conservation status and maximum recorded standard length.
• Seriously Fish: species aquarium footprint, volume, water range, filtration example, diet, shoal size, behaviour, companions and mature sexual dimorphism.
• OATA: UK rainbowfish group guidance covering aquarium size, water quality, feeding, maintenance, introduction, social care and broad compatibility.
• RSPCA: general aquarium welfare guidance covering biological filtration and the processing of ammonia and nitrite.
• Tropical Fish Co delivery policy: receipt, acclimatisation and Live Arrival Guarantee claim procedure.