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Caring for the Awesome Harlequin Rasbora

Harlequin Rasbora fishes are an old, old addition to the aquarium hobby. Introduced in the early 1900s, they have been a favorite of fish keepers for over a century.

As a result, they are pretty much all captive raised these days from the very first wild stock, which makes them far hardier than other rasboras, many of which are still imported from Asia.

Harlequin Rasbora

So if you’re looking for a nearly bulletproof, beginner-friendly fish, you can’t do much better than the Harlequin Rasbora fish!

What are Harlequin Rasbora Fish?

The Harlequin Rasbora is actually very different in appearance from many of its relatives. Rasboras tend to be small, very slim fish, while the Harlequin is deeper bodies. While not large it’s also not a small fish, either.

Rasbora fish of all kinds are members of the family Cyprinidae, which is full of dozens of aquarium favorites. Other cyprinids include Goldfish, Koi, Danios, Barbs, and many kinds of Freshwater Aquarium “Sharks!” 

Cyprinids have toothless mouths but have the ability to chew using pharyngeal teeth located in their throats! Watch any cyprinid eat and you’ll see it instantly swallow and then work at a flake or pellet with its mouth closed. That’s its chewing motion! 

A few other fish have pharyngeal teeth, including Moray Eels, though they use theirs for grabbing and swallowing prey, not chewing it.

Now that you have a basic understanding of these intriguing little cyprinids let’s take a closer look at caring for Harlequin Rasboras!

  • Scientific Name: Trigonostigma heteromorpha
  • Origin: Southeast Asia + Indonesia
  • Length: 2 inches
  • Aquarium Size: 20+ Gallons
  • Temperament: Peaceful; Schooling
  • Ease of Care: Very Easy

Similar Rasbora Types

What are Harlequin Rasbora Fish

Harlequin rasboras are better described as part of a species complex. This means that there is actually a group of very closely related fish that look similar and are genetically close to identical. 

A few members of this complex include the Pork Chop Rasbora (Trigonostigma espei). Also known as the Lambchop Rasbora, it is a smaller fish that rarely grows beyond 1 inch and has a much thinner body and purple stripe. 

Rasboras as a whole have nearly identical care requirements since they are found in the same part of the world. So anything you read here will apply to their near cousins as well!

There are a few other rasboras that occasionally enter the trade as well, including the Blackline Rasbora (Rasbora borapetensis), the Brilliant Rasbora (Rasbora einthovenii), the Fire Rasbora (Rasboroides vaterifloris), and the Orange Hatchet Danio (Chela Dadiburjori). 

We are also seeing Purple Harlequin Rasboras more frequently in the trade – these are actually a color morph of the standard Harlequin fish with a broader purple patch!

Harlequin Rasbora Care

Harlequin Rasbora Care

Harlequin fish are very straightforward and easy to care for aquarium fish! Here are the key factors in ensuring your live to be several years old!

Aquariums for Harlequin Rasboras

The first thing to consider is whether your aquarium is large enough to provide Harlequin Rasboras with enough room to swim. While they aren’t especially large fish they aren’t as small as nano fish or even livebearers.

Therefore a 10-gallon tank is a little on the small side for them; consider upgrading to a 20-gallon tank minimum if you want to keep Harlequin Rasboras. Also, remember that these are shoaling fish that enjoy the company of their own kind.

So you are looking at buying at least six Harlequin Rasboras, with more always being better.

They are found in shoals of hundreds of individuals in their native Southeast Asia and will be very shy and timid if kept in small numbers or individually.

A 20 gallon “long” is better than a 20 gallon “high,” because it provides more horizontal area for swimming. Always go with a “long” style tank whenever you can for active, fast-swimming fish like these to get the most mileage out of your water volume!

Harlequin Rasbora Aquascaping

Harlequin Rasbora fish are not very picky when it comes to your aquarium layout. Sand or gravel is irrelevant to them since they don’t dig at all. You do want to avoid filling in all of the midwater swimming area with decoration since these fish do like to race around. So give them some wide-open swimming area mixed with cover.

Live plants are great additions to any Harlequin Rasbora aquarium. In fact, I’d say you really should be keeping them as often as possible with these fish because aquatic plants offer so many benefits. For starters, they soak up ammonia, nitrite, and nitrate as fertilizer, just like your nitrifying bacteria in your biological filtration system.

Plants also absorb carbon dioxide, releasing oxygen in the process and improving aeration in your fish tank. They also encourage the growth of healthy biofilms and provide cover for small fish. This cover, in turn, makes your rasboras more likely to stay out in the open, exposed, since they know they can retreat at any moment.

Water Conditions for Harlequin Rasbora Fish

Harlequin Rasboras are originally from Southeast Asia and Indonesia and are found in highly acidic streams, swamps, and bogs.

These bodies of water are acidic to the point of being blackwater environments where the pH can be as low as 4.5. 

That said, Harlequin fish have been bred in tap water for over a century now so they will thrive and even breed in hard, alkaline conditions (pH 7.0+).

However, I do recommend keeping them in as mineral-free conditions as you can provide. You will see stronger colors and have much more success breeding them if you can lower the pH towards acidity.

There are a few ways to reduce the pH in our mineral-rich tap water. One is to buffer the water using Indian almond leaves, driftwood, and other sources of plant tannins. Another is to use liquid additives whenever you are doing water changes. 

But I’d only recommend taking these steps if you are serious about breeding Harlequin Rasboras. Otherwise they will do just fine in neutral to alkaline water chemistry!

Harlequin Rasbora fish are also very resistant to elevated levels of ammonia, nitrite and nitrate, another legacy of their decades as aquarium fish. In fact, they are some of the best fish to use if you prefer fish-in aquarium cycling. For what it’s worth, fishless cycling with ammonia is a safer, more humane way of setting up a new tank!

They do prefer warm tropical conditions, though. Unlike goldfish or danios Harlequin fish are strictly tropical fish and should not be allowed to get any colder than 74℉.

The ideal range for them is 74-82℉. And if you are trying to breed them or boost their resilience against a disease, increase the temperature to around 83-85℉!

Harlequin Rasboras also don’t need aquarium salt or any other complex additives. Just provide them with warm, clean water and you’ll almost certainly see them glow over time!

Harlequin Rasbora Tank Mates 

Choosing tank mates for Harlequin Rasboras does not need to be a complicated affair. These are peaceful community fish that will get along with just about anything!

Many small shoaling fish have the habit of nipping fins of long-finned fish like bettas, fancy guppies, or gouramis. But Harlequin Rasboras tend to keep to themselves even when presented with such tempting targets. Tiger barbs, some danios, and many tetras are much worse when it comes to fin nipping.

What’s more, rasbora fish have such small mouths that they are only a threat to extremely tiny fish, such as baby guppies or shrimp. Even fully grown Harlequin fish would not be able to eat any of their tank mates.

Stay away from any tropical fish that is extremely aggressive or predatory in any way. Harlequin Rasboras are fast enough to outmaneuver slower semi-aggressive species like bettas and paradise fish. Dwarf cichlids can also work so long as the tank is relatively spacious. Just remember that spawning dwarf cichlids could be a problem since they are as vicious as their larger relatives in defending their eggs and fry.

Typical community fish tank mates include corydoras, neon tetras, platies, kuhli loaches, sparkling gouramis, danios, and other fish in this size category.

But the best tank mates for Harlequin Rasboras have to be their own kind! They live in shoals, which are more like tightly knit groups than true schools.

A shoal of fish will break apart to do their own thing or set up little turf zones. 

However they will quickly group back up, especially when predators draw near, for the extra eyes and protection thanks to their sheer numbers. I recommend keeping them in shoals no smaller than 6 individuals, with 12 being a much more comfortable number for rasbora fish. 

Considering Harlequin Rasbora size this means keeping them in a tank closer to 30 gallons. But you will be rewarded by their beauty and constant activity!

Lastly, they also get along with most invertebrates, though you should be a little cautious since they may pick at the antennae of snails. Baby dwarf shrimp may be a meal for them – Red Cherry Shrimp breed constantly, after all. But the adults are in no danger from a Harlequin Rasbora.

Good Tank Mates for Harlequin Rasboras:

  • Livebearers, Danios, Tetras, Dwarf Gouramis, Corydoras, Kuhli Loaches, and other Peaceful Community Fish
  • Dwarf Cichlids, Bettas, and other semi-aggressive fish
  • Snails, larger Shrimp, and Dwarf Crayfish
  • Other Harlequin Rasboras

Poor Tank Mates for Harlequin Rasboras:

  • Large Cichlids, large Catfish, Arowanas, and other Predatory Fish
  • Extremely Aggressive Fish

Feeding Harlequin Fish

Harlequin fish are as straightforward to feed as they are to care for. Like most cyprinds they are omnivorous, feeding on both plant and animal matter.

That said, rasbora fish of all kinds are a little more on the carnivorous side, meaning they prefer insect larvae, small worms, and other tiny prey over plant based food.

I recommend making a high quality pellet food the foundation of their diet, something with plenty of whole ingredients like fish, squid, or spirulina. No corn, soy, or other junk fillers. Fluval bug bites are one of my favorite formulas since they use black soldier fly larvae and salmon as their foundation.

Supplement your prepared food with good frozen and live foods like bloodworms, daphnia, tubifex, and brine shrimp! These provide healthy fats, proteins, and roughage in the form of exoskeleton that other foods lack. If you can add in a nice vegetable-based flake formula now and again, so much the better!

If you don’t, you may see your Harlequin Rasboras grazing on soft plants now and again for vitamins. Cabomba, Elodea, and other such plants are easy targets for these omnivorous fish.

Breeding Harlequin Rasboras

Since they are so easy to care for, breeding Harlequin Rasboras is a pretty straightforward affair. Having at least six fish ensures you will get a male and female Harlequin Rasbora pair.

You’ll need to play the numbers game because they aren’t very easy to tell the sexes apart; male and female rasboras look nearly identical. 

Female Harlequin Rasboras tend to be a little chunkier, especially when they start ripening with eggs. But the difference is subtle since these fish are always a little chunky! Harlequin Rasboras spawn among plants, as you will see in this video!

In the above link, you’ll see that they do quite a bit of acrobatics in order to ensure their eggs attach to the leaves. But in a more heavily planted tank they will simply enter the plant patch and allow their eggs to drop, which are sticky and adhere to the first surface they touch.

Like most cyprinids all rasbora fish provide no parental care for their young. Once the eggs are fertilized and laid the male and female Harlequin Rasbora depart and they are all on their own. Fortunately, they hatch quickly; within 24-48 hours. 

The fry take an additional 48 hours before they need feeding as they have their yolk sac to absorb nutrients from.

Once the yolk is consumed the fry needs feeding on especially small live foods like infusoria and baby brine shrimp nauplii, both of which can be easily cultivated by hobbyists!

Frequently Asked Questions about Harlequin Rasboras

How Many Harlequin Rasboras Should I Get?

Never get less than 6 Harlequin Rasboras. They are very social and will feel isolated and afraid if kept in smaller numbers. They may even hide continually, thinking the others must be hiding since a predator is nearby.

Are Harlequin Rasboras Aggressive?

Not at all. Rasbora fish of all kinds tend to be peaceful to the point of being shy. So only keep them with either peaceful fish or semi-aggressive fish in more spacious aquariums.

How Many Harlequin Rasboras Can I Put in a 10 Gallon Tank?

I don’t recommend keeping Harlequin Rasboras in a 10-gallon fish tank because they are a little large for a tank that size. They grow up to 2 inches long and need to be kept in shoals of at least 6 individuals, which is more than the carrying capacity of a tank that size.

And they are so active that there is far too little swimming room in a tank like that. 20 gallons of water is the minimum tank size for Harlequin Rasboras, with a 20 gallon long being ideal for them.

Are Harlequin Rasboras Friendly?

Absolutely! The Harlequin fish is a peaceful, social community fish that lives comfortably with its own kind as well as other fish!

Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.

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