April 17

Harlequin Rasboras – Appearance, Feeding, Breeding and Tank Mates


Harlequin Rasbora

Harlequin rasboras are distinctively marked out from other cyprinids for their metallic color and a black patch that covers nearly half of their body. They are undemanding, peaceful, and thrive in a school of 6 –8.

They are small in size, and a 20-liter should be perfect for keeping them. Their tank should be adequately furnished with plants and should also have an efficient filtration system.

Besides their colorful display in the tank, the breeding pattern of this exciting species of freshwater fish is something to anticipate with great pleasure. And, for their simple lifestyle, you can be sure that you will not get worked up keeping them. Explore the world of these small but highly favored fish in this article as we take you through their behavioral pattern, dietary and tank requirements, breeding style, among other relevant issues.


If ‘mischief‘ or ‘evil‘ is all that comes to mind whenever the word, ‘Harlequin‘ pops up then it is likely that you have not heard about or come across the harlequin rasbora. The fish which is also known as ‘red rasbora‘ or ‘red razor‘ is a friendly and attractive creature that loves to swim together in a shoal. It goes the scientific name, Trigonostigma heteromorpha.

Harlequin rasbora is native to Singapore, Thailand, and Malaysia; often seen inhabiting slow-moving rivers. They also dwell in water bodies with a considerable amount of humic acid as we have in swamp peat forest – they get to utilize the essential mineral content present in this habitat, but it is necessary to stress the fact that harlequin rasboras do not like water with high mineral content on the whole.

As you will discover in this article, virtually everything about the harlequins follows an easy course, and this is why they are recommended for beginners.


Harlequin rasboras are smallish, not exceeding 5 cm within the 5 – 8 years they may get to live – they are another set of fish you should consider for your nano tank. Their mid-section is quite broad when compared to the mouth-end and tail-end. The caudal fin of the harlequin fish is forked and the other fins – pectoral and pelvic fins – follow the orientation that is feasible in most cyprinids.

An aspect of their appearance that cannot be missed out is the vibrant coloration that they possess. The body of this species has a hue of orange and silver, and the fins are usually lined with red.

Additionally, there is a black triangular patch that covers nearly half of its body and narrows progressively towards the caudal fin. The shape of this black patch coupled with the body outline of the harlequin rasbora is essential for distinguishing it from two other fishes in its subspecies namely, the lambchop rasbora and glowlight rasbora.

These two fishes are typically more slender, and their black patch is not as profound as that of the harlequin rasbora.


The omnivorous feeding habit of harlequin rasboras means that they are not so persnickety. Their diet can be composed of flake food, pellet, vegetables (e.g., lettuce, spinach, etc.), daphnia, tubifex, frozen/live shrimps, bloodworms, and so on.

You should ensure that you provide with a balanced supply of nutrients from these foods. And, because of the small size of their mouth, it is advisable that you break the food down into bits [before serving them].

You can feed them 2 – 3 times daily, but it is essential that you avoid putting more than they can chew within 3 minutes. Ensure that excess feeds are taken out of the aquarium as soon as possible. Again, meaty foods should not form the core of their diet as such may cause them to be overweighted and inactive.


Besides being a schooling set of fish, harlequin rasboras are also easy-going; they will not exhibit any form of aggressiveness towards their tank mates.

They are yet very playful especially when in groups; however, a harlequin rasbora may become timid and go into hiding most of the time when alone in an aquarium.

So, if you want to see the colorful display of this amazing creature more often, never leave it lonely. It is worth mentioning that though harlequin rasboras are found to swim at all water levels, their favorite level remains the middle layer.


    Harlequin rasboras are not tricky to take care of as they are not so demanding, and they are a hardy group of fish. All you need do is to ensure that they are kept in a clean and safe aquarium by doing a water change as at when due, taking care of water oxygen level, paying attention to ammonia and nitrite concentration, and get rid of excess foods before they decompose.

    To sum it up: do whatever it takes to maintain good hygiene within the aquarium. By the way; for the water change; you should replace 20 – 25% of the water every week. Plus, you may need to do water chemistry tests regularly to ascertain that the water parameters are within the range appropriate for their well-being.

    Tank setup

    Upon obtaining a 38-liter tank – which should be enough for housing 6 – 8 harlequin rasboras – the next thing to do is to decorate the tank as required; simulating their natural habitat as best as you can. Regarding the size of the tank, some aquarists may choose to go for large tanks to give the fish more room to swim.

    To start with, create a dark, sandy substrate and also incorporate rocks and gravels. The dark substrate will be great for bringing out the unique coloration of harlequin rasboras Now; you should make the tank densely vegetated by decorating it with plants such as anacharis, anubius, Java fern, and so on. These plants will be valuable for providing excellent cover from light and giving them perfect hiding spots.

    You can also add other items like driftwood, dried leaves, and bogwood to make the aquarium more homely for the harlequin rasboras. 

    Again, you will need to integrate the aquarium with a nice filtration system to help to take care of dissolved feed and other wastes, and also maintain a good oxygen level within their aquarium. And, you should not forget to affix a tank cover to keep them from falling out of the tank as a result of their playful nature.

    On the placement of the tank; an area with medium or low lighting is the best for them, and this also means that you may need to dim the light of the tank.

    Water parameters

    Harlequin rasboras can survive in a waterbody with wide-ranging characteristics; they will be fine in water with extremely soft to a moderately hard degree of hardness – 2 – 15dH. The appropriate temperature for them should be between 220C – 260C.

    The harlequin rasboras may be like most of the other hardy fishes you have come to know when you consider the temperature range and degree of water hardness, but the pH range somewhat sets them apart. The harlequins will survive in reasonably acidic water as well as one that is slightly basic; this means you can set the pH range between 5.5 and 8.0.

    A note of warning: in spite of the wide range of water chemistry they can survive in, it is advisable that you keep them under a stable condition. This is because inconsistent or fluctuating water chemistry may result in much stress which could result in the manifestation of certain diseases.

    Aquarium mates

    In making consideration for aquarium mates for harlequin rasboras, the first rule is that you should not put them together with bigger fishes as that would providing free lunch for the bigger mates. So, fishes like cichlids and angelfish are a definite no-no.

    Apart from these, aggressive fishes that are capable of tormenting harlequins should be kept at bay.

    The perfect tank mates for them are fishes of similar size and such that are evenly tempered. That said, apart from keeping them in a company of their kind; you can also pick from any of the following fishes: guppies, zebra loaches, platies, danios, cherry barbs, gouramis, etc. Again, there will be no issues keeping them with other aquatic invertebrates like shrimp, crabs, and snails.


    Breeding harlequin rasboras should not give you much trouble; you only have to follow some steps religiously. The first thing is that you determine the sexes of the fish you will be pairing up. The color of male harlequin rasboras is usually more distinctive than the females, and the triangular patch is rounder and more elongated in the male. The females are however plumper than the males, and this size difference is more evident as spawning beckons.

    Conditioning for spawning

    To induce spawning, you will have to condition the fish in separate tanks – or in different compartments of the same tank separated by a divider – feeding them consistently with live feeds (like daphnia, shrimp, and bloodworms) for about 3 – 4 weeks.

    The success of this conditioning will be reflected with the belly of the female protruding as it becomes filled with eggs while the color of the male gets darker.

    The breeding tank should be prepared while the conditioning is in progress. You should plant with broad leaves – cryptocoryne and anubias are among the best in this regard to decorate the tank. These leaves are highly essential for the reproduction process to go on smoothly; they provide a sort of shelter for the eggs that will be eventually laid.

    The water level should be kept low, let’s say about 20cm and the temperature of the water should be set at 280C while the pH range is maintained around 6.0. For breeding, harlequin rasboras require very soft water; hence the degree of hardness should be within 1dH and 3dH.

    The peaty condition attainable in their natural habitat should be maintained, and you should integrate an efficient filtration system into the breeding tank to cater to their oxygen need; the hang-on back or sponge filter should suffice in this regard.

    Breeding proper

    With the parameters set and every other thing in place; you can now introduce the male and female harlequin rasboras into the tank. The most appropriate time to do is around the evening, and the light should be dimmed out. By morning – which is their favorite spawning period – the female will be seen rubbing its belly against the broad-leafed plant – a move that signals the male that it’s time to get cozy.

    The male then courts the female by performing some dancing moves, and both of them end up lying in an inverted position, and mating thereof under the broad leaves.

    This action leads to the laying of eggs by the female harlequin rasboras, and they are eventually fertilized by the sperm released from the male. Around 6 – 12 eggs are released by the female at a time, and they are capable of laying between 100 – 300 eggs within a couple of hours on the whole. And, unlike most of the other cyprinids who are destitute of parental instinct, harlequin rasboras do show some traces of parental care – even though if it is not so advanced as you have in the ‘egg-gatherers‘.

    For one, they lay eggs that stick to the broad leaves by means of a gluey substance. Secondly, they are not very in tune with eating their eggs as some cyprinids would readily do. That said, once spawning is over, it is befitting that you take the adult fish out of the breeding tank.

    The eggs of harlequin rasboras will hatch within 24 – 48 hours and the young free-swimming fish, which is only about 0.4cm, should evolve 3 – 4 days later. At this stage, they should be fed with infusoria – this should have been prepared days before the babies finish feeding off their yolk sac – until they will be ready to consume brine shrimp or micro worms after 2 – 3 weeks.

    Feeding these young ones with a highly nutritious diet is sine qua non as the development of the characteristic harlequin fish’s coloration [as well as growth] is dependent on it.   

    N.B: Age is another factor to consider before breeding harlequins as it has been reported that their egg-laying capability diminishes with age; younger ones will be more fertile than the aged/older ones. Therefore, should your attempt to breed them not return a fruitful result after you must have followed the steps communicated above, you may need to select a younger couple. By the way; harlequin rasboras grow very fast; they reach sexual maturity in under one year.


    Let us now look at a couple of diseases that can spoil the fun for your harlequin rasboras.

    1. Cotton wool disease: Though this is not prevalent in harlequin rasboras; some aquarists have reported seeing some fluffy outgrowth on their pet. This fungal disease is familiar to most freshwater fish, and it is known to cause an affected harlequin to breathe rapidly and also pick up lesions on the skin. Aquarium salt and specific antibiotic formulations are effective in treating cotton wool disease.

    2. Dropsy: Dropsy is another condition that may cause your harlequin some problem. It could result from certain microbes which could infect the fish upon the breakdown of its immune system; this breakdown can result from injuries or excessive stress.

    Swelling, particularly around the fish’s abdominal region, is a common symptom of this condition. The harlequin suffering dropsy experiences lethargy; loses appetite and withdraws itself from the company of others; preferring to stay at the bottom-most of the time.

    3. Velvet disease: Harlequins are quite susceptible to velvet disease, a condition that could also manifest due to the immune system being compromised. When infected by the disease-causing organism, harlequin rasboras will have their metallic coloration subdued by a dusty brownish color. Additionally, their fins may become clamped to the body, and they may suffer labored breathing and lethargy. The fish infected by the parasites will also readily rub its body against any hard surface or object it comes across in the tank.

    4. Fin rot: Fin rot is a known disease among many aquarists keeping aquarium freshwater fishes. And, it happens that it is one disease that is somewhat prevalent to harlequin rasboras. At the onset of this disease, the fish loses its coloration – which fades off gradually – develops inflamed patches on the fins, and the fraying of fins is observed.

    5. Neon tetra disease: Yes, you read it right, but neon tetra is not a disease that affects only the fish it is so named after; harlequin rasboras [as well as some other fishes] are prone to this disease. This disease can cause your harlequin to feel uneasy [especially at night], and also make it difficult for it to swim about as usual. Furthermore, it loses color; ceases from living its normal schooling lifestyle, and may experience fin rot as the disease progresses. The mortality rate of neon tetra disease is quite high as there has been no cure to take care of it. So, you may have to do some damage control by euthanizing the stricken fish before infection spreads.

    All I have given you here is a hint of the diseases that can down your harlequin – there are many more. You can be dedicated to the welfare of the fish, and ensure that you do the necessary research once you notice any strange sign on the body of the fish. Treatment should commence as soon as possible where appropriate.

    Their metallic color means glamor in your aquarium and that they are easy to make them a delight to every aquarium enthusiast; harlequin rasboras are truly worth having in and around the home. Getting one for your home aquarium should not be a problem as they are vastly available.


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