Harlequin Rasboras are great fish to keep, their schooling behaviours, ease of care, peaceful nature and bright colours make them superb additions to any community planted tank.
But can they fit into coldwater setups? What temperature can Harlequin Rasboras be kept at? And in what ways are they affected at different temperatures?
Can Harlequin Rasboras live in cold water?
Harlequin Rasboras are tropical fish, they do not live in cold water and come from waters that regularly measure above 22C / 71F.
They are sometimes considered as temperate fish because they can go a little cooler than most other tropical fish.
However, this is not the best way to describe them, as they will struggle to survive in waters cooler than 18C / 64F.
They are tropical fish, although they generally prefer to be in the cooler range for the majority of their lives.
It is true that their natural habitat does on some occasions receive dips in temperature that go below 18C.
The large bodies of water they are found in however, are able to hold onto the heat quite well, and temperature change is often slow and gradual, allowing the fish to adjust.
For these reasons, we recommend keeping your Harlequins at standard tropical temperatures.
Can Harlequin Rasboras live in a tank without a heater?
For some, it is possible to keep Harlequin Rasboras without the use of any heating equipment, although it is heavily dependent on the setup, location and time of year.
For some people, the outside ambient temperature is already enough to keep the tank warm enough for the fish to do well.
However, we still always recommend having a heater, even if you do live in a very warm climate, as there will be times in the year when the temperature does drop below the safe range.
Having a heater is like having a backup life support system for your fish, it is better to have one ready for use when times get cold, than to not have one and subject your fish to temperature fluctuations they are not built to cope with.
How long can Harlequin Rasboras survive without a heater?
Depending on the external ambient temperature, your Harlequin Rasboras may survive a couple weeks without a heater, a couple of days, or a couple of hours.
Harlequin Rasboras are fairly hardy when it comes to temperature drops, especially when compared to other tropical fish, but significant drops, where the temperature may go well below average room temp (20C / 68F) can be dangerous if the fish are exposed for a long period of time.
We would not recommend exposing your fish to temperatures below 20C for longer than 48 hours.
If the surrounding temperature is warm however, then you may get away with not having your heater powered for as long as the weather stays warm.
In summer, you may save some electricity by leaving your heaters off while it is hot outside, before turning them back on once autumn approaches and things get colder again.
Why do Harlequin Rasboras need a heater?
Harlequin Rasboras need a heater because they originate from sub tropical areas of Southeast Asia, where the annual temperatures are generally above those in temperate regions.
These fish have evolved over millions of years to live in a climate that regularly stays above 22C / 72F and so they are adapted to live in this environment.
They are not used to chilling temperatures like coldwater fish, and will die if exposed to conditions that dip below their normal comfortable temperature range for too long.
A heater allows you to better control the environment in your fish tank, and suit it to the needs of the Harlequin Rasboras, so that they may thrive in an enclosure which mimics their natural habitat.
What temperature should a Harlequin Rasbora tank be?
Harlequin Rasboras can be fairly lenient when it comes to temperature, but they do have a preferred range, which you should aim for if you want your fish to do well.
It is generally recommended that you should keep your Harlequins anywhere from 22C / 72F to 26C / 79F.
Through our experience in keeping Harlequins, we personally like to keep ours at 24C / 75F as they seem to do best and be the most comfortable at this temperature.
When it comes to attempting to breed them, we would suggest increasing this temperature to 28C / 82F gradually over a period of days.
Types of aquarium heaters for Harlequin Rasbora tanks
Any sort of aquarium heater is suitable for Harlequin Rasboras, it is all down to the setup you use, and the tank size they are being kept in.
For some, a large heavy duty heater is necessary to keep their huge 10ft tank up to temp.
For others, a small and simple glass aquarium heater stuck to the back of their 20 gallon tank is all that’s needed to keep the fish comfortable.
The fish themselves put no restrictions on the type of heating you use, so determining the size and type of heating equipment you use is solely based on how you set up the tank.
If you have a large tank with a sump, we recommend using an appropriately sized glass or titanium heater, of a high wattage, preferably at a level which matches the literage of the tank.
If your tank, including the sump, has a total volume of 300 litres, then you need a 300 watt heater to keep it warm.
Titanium heaters are great, they are heavy duty, resistant to breakage, reliable, and keep the water at the preferred level extremely efficiently.
However, for most people who have a small 10 or 20 gallon aquarium, a standard glass 50w – 100w heater will work just fine, and is very easily incorporated to a range of different setups.
FAQs about heating Harlequin Rasbora tanks
How do you test the water temperature in a Harlequin Rasbora tank?
There are all different types of thermometers out there on the market today, and most all can be used to test the temperature in different ways.
However, they vary in levels of quality, and some are generally much better than others at providing an accurate and reliable reading.
Some of the best thermometers out there come in the form of digital testing equipment, such as TDS meters, which often have a probe which will detect and show the temperature of the water on a digital screen.
TDS meters are very easy and quick to use, and are one of the most efficient ways of ensuring your tank is in the right, stable condition.
Do Harlequin Rasbora fry need a heater?
It is paramount that your Harlequin Rasbora fry have a heater, as they must be kept at very warm temperatures in order to survive and properly develop.
The eggs, wigglers and fry have to be kept warm, they need to stay above 25C / 77F at all times.
Only once they become well developed juveniles or sub adults, the temperature can be slowly decreased, but in those early developing stages, they have to be kept warm.
What happens to Harlequin Rasboras if the water is too cold?
As the temperature of the surrounding water decreases, your Harlequins will become less and less active, as they attempt to preserve energy.
Gradual temperature dips are natural for Harlequins, as things like season changes, night and day differences and weather conditions regularly affect them.
However, these changes are slow, as the large volume of water heats up and cools down over a longer period of time.
In an aquarium, the temperature is less stable, and a quick drop can shock your fish, which causes stress, organ damage and disrupts normal bodily functions.
If exposed to temperatures lower than 18C / 64F for too long, the fish can succumb to organ failure, and eventual death, which is why it is so important to have a heater, as a backup.
What happens to Harlequin Rasboras if the water is too warm?
As the temperature in your Harlequin Rasbora tank rises, the fish will become more active, as their metabolism is increased with the warmer water.
They will need to eat more, as they will burn fat much faster, and will be much more active than normal.
They may also show spawning behaviour, and may be more aggressive as they become irritable.
Once the temperatures get too warm, the fish start to suffer from heat stress, and will be seen at the top, gasping for air and rapidly flapping their gills.
This usually occurs at temps past 32C / 90F.
Over time they will suffer from organ failure and will eventually die, unless the temperature can be brought back down to safe levels.