Cairns Rainbowfish - Cairnsichthys rhombosomoides
Cairns Rainbowfish - Cairnsichthys rhombosomoides

Cairns Rainbowfish - Cairnsichthys rhombosomoides

Cairns Rainbowfish was scientifically described by Nichols & Raven in 1928 and given the name Cairnsichthys rhombosomoides. The name is derived from the geographical location of the fish; it lives between Cairns and Innisfail near the Pacific coast of northern Queensland in Australia.

Even though Cairns Rainbowfish was described during the late 1920’s, it didn’t enter the aquarium trade until the 1980’s. It is still not a very commonly kept fish, partly because it is less flamboyant than many other Rainbowfish species. It has also gained a reputation for being tricky to keep and is not recommended for beginners. 

Cairns Rainbowfish can reach a length of 8.5 cm (3.3 inches), but most specimens stay around 6 cm (2.4 inches). Compared to other rainbows, these fishes are rather slim bodied. They are fairly unexciting when it comes to colouration and are camouflaged by a dull yellow-green colour. The belly is white and the body is decorated with a thin black mid-lateral stripe. The males display a yellow margin on dorsal and anal fins.

Geographical distribution, habitat and conservation
As mentioned above, Cairns Rainbowfish lives near the Queensland Pacific Coast between Cairns and Innisfail. In this region you will find the Bellenden Ker Range, a congregation of peaks that rises from the coastal plain. The typical Cairns Rainbowfish habitat is the small streams that drain the Bellenden Ker Range, especially creeks in the lower parts of the peaks. You can hardly find any members of the genus Cairnsichthys in the nearby flood plains. The creeks where you can find Cairns Rainbowfish are clear and contain both rapid sections and rocky pools. They are never deeper than 3 metres (10 feet) and sometimes the water depth is just a few centimetres (roughly 1 inch). Aquatic plants are very rare.  

Since the geographical distribution of the genus Cairnsichthys is so limited, even limited habitat destruction could pose a threat to them. Fortunately, much of the rainforest in the Bellenden Ker Range has been turned into a National Park.

Keeping Cairns Rainbowfish in aquariums
The natural habitat of Cairns Rainbowfish is clear rainforest creeks with both rapids and rocky pools and you should therefore try to create similar conditions in the aquarium. A strong current is appreciated, but you must also provide the fish with sheltered spots in the aquarium where it can rest. The oxygen content must always be high. The bottom should ideally be covered in gravel, pebbles and rounded rocks. Aquatic plants are not mandatory since they are unusual in the natural habitat of these fishes. Adding plants or a spawning mop is however a good idea if you want your fish to breed. Cairns Rainbowfish is an active swimmer and should ideally not be kept in aquariums smaller than 100 cm (slightly more than 3 feet).

One of the reasons why the Cairn Rainbowfish is viewed as a sensitive species is that his fish does not cope well with handling and transport. If you manage to keep it alive during the initial period, it will become much sturdier and less susceptible to illness. It is however always best to keep the environment really stable in the aquarium when keeping Cairn Rainbowfish. Keep the water temperature within the 20-25 degrees C (68-77 degrees F) range and the water slightly alkaline, from pH 7.2 to 7.6. Small and frequent water changes are better than big ones; never change more than 20% of the water during each change. 

Cairns Rainbowfish is not a finicky eater in the aquarium and will readily accept most food types. It is important to keep it on a varied and nutritious diet. 

Breeding Cairns Rainbowfish
Cairns Rainbowfish has been bred in aquariums. Keep your fish in an optimal environment and feed them meaty food, e.g. live brine shrimp and chopped fish fillets to get them into breeding condition. Provide the fish with a suitable spawning spot, e.g. bushy plants or spawning mops. After the spawning, the eggs will normally hatch within a week. Feed the fry newly hatched brine shrimp or similar. 

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