Cairns Rainbowfish
Cairnsichthys rhombosomoides
The Cairns Rainbowfish is a critically endangered species native to the Wet Tropics of Queensland, Australia. Unlike many rainbowfish, this species inhabits clear, fast-flowing, shaded rainforest creeks with rocky bottoms and virtually no aquatic plants. They are active, peaceful schooling fish that require highly oxygenated water and plenty of horizontal swimming space.
Cairns Rainbowfish At a Glance
Cairns Rainbowfish Tank Requirements
Will Cairns Rainbowfish fit in your tank?
Care guides tell you what Cairns Rainbowfish needs, the next question is whether your tank can safely provide it. Test Cairns Rainbowfish against your tank size, maintenance needs, and the fish you already keep before you buy.
Cairns Rainbowfish Water Parameters
Cairns Rainbowfish Temperament & Compatibility
Cairns Rainbowfish Varieties
Each variety inherits the species defaults above. Only differences are highlighted.
Standard / Unsexed
Typical commercial form, usually sold as juveniles before distinct sexual characteristics develop. Must be kept in an active shoal of 6 or more.
Differs in 1 trait from base species
Also known as: Juvenile
Male
Males develop a deeper body as they mature and display distinct yellow margins on their dorsal and anal fins. They display their best colors when competing for females.
Differs in 1 trait from base species
Also known as: Adult Male
Female
Females are generally slimmer and have more subdued coloration, lacking the bright yellow fin margins of the males. Keeping a ratio of 2 females per 1 male reduces harassment.
Differs in 1 trait from base species
Also known as: Adult Female
Cairns Rainbowfish Care Guide Summary
The Cairns Rainbowfish, scientifically known as Cairnsichthys rhombosomoides, belongs to the Rainbowfish family group. Adults reach a maximum size of approximately 8.5 cm and have a fusiform body shape with a moderate growth rate. Classified as peaceful, this species primarily occupies the mid-level and surface zones of the aquarium and produces a low level of biological waste.
To thrive, the Cairns Rainbowfish requires a freshwater only aquarium with a minimum volume of 110 litres and a tank length of at least 90 cm. Water temperature should be maintained between 20°C and 26°C, with a pH range of 6.5–7.6 and a general hardness of 2–10 dGH. A high water flow is preferred.
Due to its specialist needs, the Cairns Rainbowfish is best suited for fishkeepers who can provide a carefully planned setup. This species is available in 3 varieties: Standard / Unsexed, Male, and Female. Each may differ slightly in size, temperament, or care needs, so it is worth checking the specific requirements of the variety you plan to keep.
Cairns Rainbowfish Habitat Setup
When setting up an aquarium for the Cairns Rainbowfish, a high water flow best suits their natural preferences. Because this species occupies the mid-level and surface zones, the aquarium layout should prioritise adequate surface area with floating cover to make them feel secure near the waterline and open swimming space in the centre of the tank for free movement. Useful additions include smooth gravel or rounded substrate to protect their sensitive barbels, driftwood, which can serve as both a hiding place and a dietary supplement, and leaf litter or blackwater tannins to replicate their natural soft-water habitat. Keepers should note that a tight-fitting lid is essential, as this species is a known jumper.
Cairns Rainbowfish Compatibility Notes
Classified as peaceful, the Cairns Rainbowfish is a schooling species that feels most secure in a sizeable group, where natural shoaling behaviour reduces stress and brings out their best colours. When choosing tankmates, their energetic swimming style may stress more sedentary or timid species. Selecting tankmates that favour different water layers from the mid-level and surface zones can help reduce competition and territorial overlap.
Who Cairns Rainbowfish Is Actually For
Cairns Rainbowfish is best for a fishkeeper who wants a species that can settle into a calmer community setup and can provide a keeper who can commit to a proper group of at least 6. It makes the most sense for a hobbyist who will actually build around its needs, especially smooth gravel.
The most common mistake is buying Cairns Rainbowfish for looks first and only later discovering how its adult size, behavior, or bioload changes the whole tank plan. Another frequent problem is underestimating its social needs. A token pair or trio often behaves very differently from a real group.
Cairns Rainbowfish Recommended Tank Sizes
Standard aquarium sizes that meet the minimum volume and length requirements for Cairns Rainbowfish.
30 Gallon Breeder
40 Gallon Breeder
55 Gallon Standard
60 Gallon Breeder
75 Gallon Standard
90 Gallon Standard
Show 10 more tank sizes
Frequently Asked Questions About Cairns Rainbowfish
How big does Cairns Rainbowfish get?
Cairns Rainbowfish (Cairnsichthys rhombosomoides) typically reaches a maximum adult size of approximately 8.5 cm. They have a fusiform body shape and a moderate growth rate.
What tank size does Cairns Rainbowfish need?
A minimum aquarium volume of 110 litres and a tank length of at least 90 cm are recommended for Cairns Rainbowfish. This ensures enough swimming space and helps maintain stable water conditions.
What water parameters does Cairns Rainbowfish need?
Cairns Rainbowfish thrives in freshwater only conditions with a temperature range of 20–26°C, a pH between 6.5 and 7.6, and a general hardness of 2–10 dGH. A high water flow is preferred.
Is Cairns Rainbowfish peaceful or aggressive?
Cairns Rainbowfish is classified as peaceful with an aggression score of 2 out of 10. They are generally well-suited to peaceful community tanks.
How many Cairns Rainbowfish should be kept together?
Cairns Rainbowfish is a schooling species and should be kept in groups of at least 6. Larger groups help distribute aggression and encourage natural schooling behaviour.
Does Cairns Rainbowfish have any special tank requirements?
Yes. Cairns Rainbowfish benefits from smooth gravel (sensitive barbels), driftwood (digestion/hiding), and leaf litter/blackwater in the aquarium. These features help replicate their natural habitat and promote overall health and well-being.
Does Cairns Rainbowfish need a lid on the tank?
Yes. Cairns Rainbowfish is a known jumper, and a tight-fitting lid or cover is essential to prevent escape. Even a small gap can be enough for a determined fish to leap out.
Useful setup supplies
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Reviewed against Guidarium care, stocking, and compatibility standards. Read the editorial policy.
- Last reviewed
- May 3, 2026
- Last updated
- May 3, 2026
- Issues or corrections?
- Contact the editorial team
Top Tank Mates for Cairns Rainbowfish
These fish are strong compatibility matches for Cairns Rainbowfish based on water preferences, temperament, adult size, and tank behavior.
Spotted Hillstream Loach
Gastromyzon punctulatus
Ideal Tank MatesWhite Cheeked Goby
Rhinogobius duospilus
Ideal Tank MatesShovelnose Sturgeon
Scaphirhynchus platorynchus
Ideal Tank MatesReticulated Stingray (Teacup Ray)
Potamotrygon orbignyi
Ideal Tank MatesRaspy River Stingray
Potamotrygon scobina
Ideal Tank MatesTandanus Catfish
Tandanus tandanus
Ideal Tank MatesPlants That Suit Cairns Rainbowfish
These plants pair well with Cairns Rainbowfish based on shared water preferences, layout fit, and how likely they are to hold up in the same aquarium.
African Water Fern
Bolbitis heudelotii
Boivin's Aponogeton
Aponogeton boivinianus
Capuron's Aponogeton
Aponogeton capuronii
Madagascar Lace Plant
Aponogeton madagascariensis
African Onion Plant
Crinum calamistratum
Afzel's Anubias
Anubias afzelii
Fish Similar to Cairns Rainbowfish
Murray River Rainbowfish
Melanotaenia fluviatilis
A robust and active schooling fish native to the cooler waters of the Murray-Darling basin in Australia. Unlike many tropical rainbowfish, it thrives in subtropical or unheated aquariums at normal room temperatures. They are peaceful but extremely fast swimmers, requiring long tanks with plenty of open swimming space.
Western Rainbowfish
Melanotaenia australis
An energetic, colorful schooling fish native to Northwestern Australia. They display prominent lateral zigzag lines and bright red or orange fins. Known for their active nature, they require plenty of horizontal swimming space and should be kept in schools to reduce stress and encourage males to display their best colors. They are boisterous feeders that may outcompete shy fish.
Crimsonspotted Rainbowfish
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Misool Rainbowfish
Melanotaenia misoolensis
An active, schooling species endemic to the limestone karst streams of Misool Island in West Papua, Indonesia. They are found in moderately fast, clear waters flowing through primary rainforests. Like many rainbowfish, they are slow to mature but develop stunning colors and deep bodies as adults. They require hard, alkaline water and ample horizontal swimming space.
Red-Finned Rainbowfish
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Bulolo Rainbowfish
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A rare and highly active rainbowfish native to the fast-flowing mountain streams and white-water rapids of the Markham and Ramu river systems in Papua New Guinea. They possess a silvery body with a distinct black streak on the upper and lower edges of the caudal fin. Because of their specialized natural habitat, they require highly oxygenated, fast-flowing water and alkaline conditions to thrive in an aquarium.