Aquarium Chillers

Aquarium chillers play an extremely important role in keeping the water temperature  in our marine tanks under control. Keeping the temperature stable and within range is absolutely vital if you are going to have long term success with SPS or LPS corals.

Reef aquarium literature tell us that the ideal temperature range for a reef tank with tropical marine fish and corals is between 24°C and 29°C. But from my experience, the absolute best temperature range which will allow you to keep ANY kind of coral is between 25°C to 27°C.

If the ambient room temperature around your tank exceeds 26°C, especially during the summer months, it is likely that you will need an aquarium chiller. Heat from equipment like aquarium lights and pumps will cause water temperature to easily rise to 30°C or higher. This temperature is especially lethal to corals if kept at this temperature for several days.

It should also be noted that dissolved oxygen levels in water will drop as temperature increases.  Also at 29°C and above, fish metabolisms increase and shorter lifespans can be expected.  

aquarium chillers

Arctica aquarium chiller in operation


Aquarium Chillers And Temperature Stability

Our aquarium inhabitants evolved in the vast ocean where temperatures are relatively stable.  Even on the shallow reefs, with the sun beating down at midday, there is usually not more than a 3 or 4°C swing between daytime and night time temperatures. Temperatures are even more stable at greater depths.

But it should be noted that while temperature swings of 3 or 4°C are fine for corals on the shallow reefs in the wild, temperature swings of more than 2°C within a 12 hour period are likely to stress corals when they are in the confines of our reef aquariums. The reasons for this are not entirely clear, but it definitely has something to do with the tiny volume of water (relative to the size of the ocean) that are our reef tanks.

This is another key advantage that aquarium chillers offer — keeping the temperature in the reef tank stable. In fact, with a properly sized aquarium chiller — or, should I say, oversized aquarium chiller (as we will discuss below) — it is possible to keep our aquarium temperature more stable than the reefs in the wild.

Sizing Aquarium Chillers

Aquarium chillers are usually rated in terms of horsepower (HP) or sometimes, British Thermal Units (BTU).

To get an idea of the chiller size you will need, calculate the total gallonage of your main display tank together with your sump using the formula mentioned on page on Aquarium Tank Size | Length, Width and Height.

aquarium chiller

Always choose the correct size chiller for your tank

After you have calculated your total water volume, check the water temperature with all lights and equipment running. Subtract the difference between the current temperature against the optimal temperature you would like to keep your aquarium at. For most tanks, the difference will be 3°C or 4°C at most.

Aquarium chiller manufacturers provide specifications on the different chiller sizes they make as well as recommended tank sizes (in gallons) for each chiller model. Some manufacturers will even specify the pull-down capacity for their chillers. Pull-down capacity is the number of degrees the chiller will be able to pull the temperature down in an aquarium relative to the gallonage of that aquarium.

For example, at an ambient temperature of 30°C (86°F), a 1/3HP chiller might be able to pull-down the temperature by 27° for a 130 gallon tank, 18° for 260 gallons and 9° for 425 gallons.

Based on these factors, it is always recommended that you purchase an aquarium chiller that is one size larger (in terms of horsepower), than the minimum required for your size tank. Choosing a chiller one size larger also means that it will not have to work as hard to keep the water temperature at an optimum.

Types Of Aquarium Chillers

In-Line Chillers

In-line aquarium chillers are the most popular because they are so efficient.

In-line chillers require a pump powerful enough to send water from the sump to the chiller and back again in a continuous loop.  Chiller manufacturers will also specify the correct flow rate of the pump to be used.  If the flow is too high, the water will not have enough contact time with the metal cooling coil in the chiller.

The metal coil in the chiller is cooled by a refrigerant.  As the water flows through the coil, it is gradually cooled through a process of heat exchange.  And never run your in-line chiller without water flowing through it as this can permanently damage the chiller!

Always have the chiller pump run from a compartment of the sump near the water returning from the display and out to the chiller.  Cooled water coming back from the chiller should be directed back into a compartment in the sump that is just before the main return pump to the main display. If the cooled water from the chiller is directed into the same sump compartment as the main return pump, microbubbles will likely be formed and sent back to the main display.

Some hobbyists prefer the return from the aquarium chiller to be directed in the main tank instead of to a later compartment of the sump.  I feel that this setup is a recipe for disaster.  If the main return pump from the sump to the display tank were to fail for any reason, the chiller pump will continue to send water to the chiller and back to the main display.  This has the potential to drain the sump, eventually causing no water to be sent to the chiller, permanently damaging it.

Drop-In Aquarium Chillers

Drop-in aquarium chillers utilize a long titanium cooling coil that is placed in the aquarium sump.  The titanium coil is filled with refrigerant, waterproof sealed and connected to the chiller’s compressor unit with a thermostat which works in very much the same way as an in-line chiller.

Drop-in chillers were all the rage for a while with many hobbyists ‘upgrading’ from their in-line models.  In actuality, drop-in chillers are not as efficient as in-line chillers as they require more energy to cool the water.

Some custom-made drop-in chillers are even powered by conventional air-conditioner compressors.

Thermo-Electric Chillers

Thermo-electric aquarium chillers work on the principle that if a current is sent through two dissimilar conducting materials, heat is absorbed on one side while heat is rejected on the other side.    Also known as the Peltier Effect, one side of a thermo-electric plate becomes cool while the other side becomes hot.  By insulating the cool side from the hot side, the cool side can be directed into the aquarium, cooling the water.

Thermo-electric chillers work only on small aquariums of around 15 gallons at most.

What Else To Look For When Buying An Aquarium Chiller

Power Consumption and Current Draw

While the horsepower of an aquarium chiller gives us an idea of the proper size chiller to get, BTUs and power consumption in watts will give a clear indicator as to the energy required to run a chiller 24/7.

Like an air-conditioner, aquarium chillers run idle, drawing little power until the built-in thermostat tells the chiller to kick-in and start cooling the water. It is during the cooling phases that the most electrical energy is drawn.

Many beginners make the mistake of buying an undersized chiller since the wattage and power consumption rating will be lower, making it seem like it will be more economical to run.

But one very important thing to be aware of is that an undersized chiller will ‘kick-in’ more often.

And the number of times, and the length of time that an undersized chiller kicks-in as it struggles to cool the water means that it is drawing a lot more energy than a larger capacity chiller that is idling most of the time! This will have a huge impact on our electricity bill!

Running a slightly oversized aquarium chiller means that it will kick-in less often since it is more efficient at cooling the water to the desired temperature. A higher capacity chiller is also not working as hard, meaning that it will give more years of faithful service.


Modern aquarium chillers always use ecological R134a refrigerant, so this is mostly a non-issue.  US Federal law in 1992 required that R134a replace the older refrigerant types that were damaging the ozone layer.

Titanium Heat Exchangers

For saltwater aquarium systems, always choose a chiller with a titanium heat exchanger.

Titanium is more costly but is impervious to saltwater and corrosion and will last forever. Stainless steel heat exchangers will corrode over time, and copper exchangers are a definite no-no for marine tanks as they can poison your livestock.

Aquarium chillers with titanium heat exchangers are more expensive but they are a worthy investment for the long term.

Aquarium Chillers | Placement

Aquarium chillers need to be placed where air can flow freely around it. Most importantly, always ensure that the front vent of the chiller is completely unrestricted and and that there is at least an 8-inch gap between the back of the chiller and the wall.

For best performance, aquarium chillers should be also placed away from external heat sources such as heaters, refrigerators or aircon compressors. The heat from these appliances will hinder the efficiency of the chiller.

And whatever you do, avoid placing the chiller in the cabinet below the aquarium. While it is a very convenient solution to finding a place for your chiller, this will seriously restrict airflow and recycle heat back into the chiller causing it to work harder than it needs to. Not to mention heating up the water in your aquarium sump!

Poor ventilation and restricted airflow around your a chiller increases power consumption while shortening its lifespan.

Aquarium Chillers | Maintenance

Aquarium chillers will need routine maintenance such as removing the front grille and vacuuming the build-up of dust. This is easy to perform and greatly helps your chillers performance.

Once every six months or so, the pump for the chiller should be removed and cleaned of mulm and calcium buildup. A clogged or failed pump will not supply water, causing the heat exchanger coil to freeze up and crack, ruining your chiller and voiding the warranty. Titanium coils in aquarium chillers are very expensive to replace!

Once every year or so, aquarium chillers will need to be flushed with vinegar to remove any calcium buildup. This is especially if you see a marked decrease in the efficiency of your chiller. This process involves pumping an acid or vinegar based solution through the chiller’s heat exchanging coil, in the opposite direction of normal water flow.

If you are not able to do this yourself, see if your local aquarium store or chiller dealer offers such a service.

Recommended High Quality Aquarium Chillers

So what are the best aquarium chillers on the market today?

For my money, nothing beats chillers made by TECO or Arctica.  These two companies have been making in-line chillers for years and they stand behind their products.  Both companies also insist that the dealers representing them are able to solidly support their products when it comes to replacement parts, repairs and after-sales service.

Links to more Saltwater Aquarium Setup pages:

Aquarium Tank Size | Length, Width And Height

Ideal Location For Your Saltwater Aquarium In Your Home

Ready-Made Aquariums

Drilled Overflows Vs. Siphon Overflows

Aquarium Stands

Why Beginning Hobbyists Should Avoid Starting A Nano Aquarium

How To Setup A Nano Tank

Aquarium Chillers

Aquarium Sump



Aquarium Covers | Put A Lid On It!

Cooling Fans


What You Don’t Need For Your Saltwater Aquarium!

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