Last Updated: September 25, 2026
Chiller size is not a gallon number or a fixed horsepower mapping; it is a continuous cooling rate that offsets your tank’s heat gain. So what size aquarium chiller do you need comes down to converting volume, temperature drop, and continuous heat into a rate, then matching a class under real conditions.
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By David Nguyen
Quick answer: Our top pick in 2026 is the <=10-gallon nano — our #1 rated choice. See the full ranked comparison, alternatives and buying advice below.
How to Size the Aquarium Chiller You Need, By the Numbers
Sizing follows a short sequence of numbers, shown in the table below. Measure total volume, set the temperature drop, convert that to a heat quantity and then a rate, and add the continuous heat your equipment supplies, because the resulting rate, not gallons alone, points to the right chiller class.
| Input | Result | Note | Exception |
|---|---|---|---|
| <=10-gallon nano | Nano-class unit (e.g. Nano E10G) | 9-11 F drop; 78 GPH feed | Hot room shrinks the drop |
| <=20-gallon tank | Larger nano (Nano E20G) | 118 GPH feed | Reef lighting adds load |
| 10-40-gallon tank | Active Aqua 1/10 class | 1,020 BTU/h; 132-396 GPH | Rating depends on conditions |
| ~75-gallon system | 1/10 HP compressor (DBA-075) | 240-960 GPH feed | Not a fixed HP mapping |
Start From Total Volume
Volume anchors the calculation, so include the sump and plumbing, not just the tank. A 55-gallon system, for instance, is the starting figure for the heat-quantity step, and manufacturer gallon ratings themselves depend on equipment and the pull-down delta, so a rating is a guide, not a fixed promise for every setup.
Read gallon ratings as conditional. A unit rated to a gallon figure assumes certain conditions and a modest drop, so a hot room or heavy lighting reduces its effective reach, which is why volume feeds a rate calculation rather than settling the choice by itself. Carry the true total volume into the next step.
Set the Temperature Drop
Set the drop from your summer peak to your target in Fahrenheit, since that ΔT drives the heat quantity. A 55-gallon system needing a 5-degree drop is the basis for the energy figure, and measuring the real peak rather than guessing keeps the whole size estimate grounded in your actual conditions.
An optimistic drop undersizes the unit. If the tank actually climbs higher than you assumed, the chiller faces a bigger ΔT than planned and runs short, so log the true summer peak under full lighting. The drop, paired with volume, sets the heat quantity the next step converts into a rate.
Convert to the Correct Energy
Turn volume and drop into a heat quantity: gallons times 8.3 times ΔT gives BTU of energy. A 55-gallon system with a 5-degree drop is about 2,282 BTU, the heat to remove to reach setpoint, which is a quantity, not a rate, so it must be divided by a time to become useful for sizing.
Over an idealized 5-hour pull-down, that 2,282 BTU is roughly 457 BTU per hour before the continuous load, as the note and exception columns reflect. Keep energy and rate distinct here, because labeling the raw BTU quantity as a per-hour rating is the single most common sizing mistake to avoid.
Add the Continuous Heat
Finally, add the steady heat from lighting and pumps to the pull-down rate. The chiller must hold the tank against ongoing heat gain, so the required rate is the pull-down rate plus that continuous load, which is why the table’s exceptions note that reef lighting and warm rooms push the requirement higher.
Include this before choosing a class. A unit sized only for the initial pull-down will run constantly once lights and pumps are running, so factor the continuous load into the rate. That combined rate, read against the table, is what points to a nano, mid-size, or compressor class for your tank.
Adjust for Your Situation
The table gives a baseline, but real tanks vary, so adjust before buying. Account for a nano’s fast swings, a mid-size tank’s heat load, a larger system’s capacity needs, and a hot room’s added continuous heat, so the class you choose fits your conditions rather than a textbook example.
Adjust for a Nano Tank
A nano’s small volume swings fast, so match a nano-class unit to its true drop. A Nano E10G suits up to about 10 gallons with a roughly 9-to-11 degree Fahrenheit pulldown and 78 GPH of feed flow, and a Nano E20G covers up to about 20 gallons at 118 GPH, sized to small loads.
Do not oversize a nano. A large unit short-cycles on a small volume, wasting energy, so a right-sized nano system that cycles is better than a compressor forced onto a tiny tank. Read the drop at your true volume and ambient, since a hot room shrinks even a nano unit’s effective cooling.
Adjust for a Mid-Size Tank
A mid-size tank leans on a published rate. The Active Aqua 1/10 covers roughly 10 to 40 gallons with a 1,020 BTU-per-hour rate and 132 to 396 GPH of feed flow, so match your calculated rate to that figure and confirm the flow range fits your plumbing before choosing it.
Let heat load, not just gallons, decide within the range. A brightly lit or heavily pumped mid-size tank sits toward the top of the class, so size to your rate with margin, and keep the feed pump in the unit’s GPH band so it can actually deliver the rated cooling.
Adjust for a Larger System
A larger system needs a compressor class read against conditions. A JBJ DBA-075, a fractional-horsepower unit, is rated to roughly 75 gallons with 240 to 960 GPH of feed flow, but that gallon figure is not a fixed horsepower mapping; it depends on equipment, drop, and ambient, so treat it as conditional.
Avoid the static assumption that a size always equals a horsepower. A larger tank in a hot room or under heavy lighting may need more than a gallon rating implies, so size to your calculated rate and use the class as a shortlist, confirming feed flow and ambient before committing to a unit.
Adjust for a Hot Room
A warm room raises both the continuous heat load and the strain on the unit. Manufacturer ambient ranges exist for a reason, so a chiller in a hot space needs more capacity than the same tank in a cool one, and its effective reach shrinks as the room temperature climbs in summer.
Add the room’s heat to your rate and ventilate the unit. Include ambient in the continuous load, keep the chiller out of a sealed warm cabinet, and give it cool intake air, since a hot, poorly vented spot can make even a correctly sized unit run constantly on the hottest days.
Frequently Asked Questions
The size question splits into a few common cases, and each answer gives a rate-based approach rather than a gallon shortcut. These cover sizing a 20-gallon tank, a 40-gallon tank, and a 75-gallon system, and whether horsepower alone can ever pick the right size for you.
What Size Chiller for a 20-Gallon Tank?
A nano-class unit usually fits: a Nano E20G is rated to about 20 gallons with a roughly 9-to-11 degree Fahrenheit pulldown and 118 GPH of feed flow. But confirm with a rate, since a brightly lit or warm-room 20-gallon tank may need more cooling than the gallon rating alone suggests.
Compute volume, drop, and continuous load into a rate, then match it. If your calculated rate sits within the nano class under your conditions, the Nano E20G suits it; if a hot room pushes the requirement higher, step toward a unit with more capacity rather than trusting the gallon figure by itself.
What Size Chiller for a 40-Gallon Tank?
A 40-gallon tank often lands in the Active Aqua 1/10’s 10-to-40-gallon range, with its 1,020 BTU-per-hour rate and 132 to 396 GPH of feed flow, but sit it against your calculated rate. Heavy lighting or a warm room can push a 40-gallon requirement toward the top of that class.
Match the rate and the flow, not just the gallons. Confirm your continuous load and drop, translate them to a per-hour rate, and check the unit’s rated cooling clears it with margin, then verify the feed pump holds the unit’s GPH range so it can deliver that cooling in practice.
What Size Chiller for a 75-Gallon System?
A larger system points to a compressor class such as the JBJ DBA-075, rated to roughly 75 gallons with 240 to 960 GPH of feed flow, but that figure is conditional, not a fixed rule. Equipment, temperature drop, and ambient all shift the real requirement above or below the gallon number.
Size by rate and confirm conditions. Calculate your cooling rate including continuous load, use the 75-gallon class as a shortlist, and verify feed flow and ambient fit, since a hot room or heavy reef lighting can push a 75-gallon system past what the rating implies on its own.
Can Horsepower Alone Pick the Size?
No. There is no fixed rule that a given gallon count always equals a set horsepower, and horsepower is not a rate you can compare across brands, so using it alone leads to mis-sizing. Horsepower helps shortlist a class, but the choice must rest on a calculated cooling rate.
Turn your load into a rate and match it. Convert volume and drop to energy, add a time basis and the continuous load for a per-hour rate, then compare that to a unit’s documented cooling. Treat horsepower and gallon ratings as rough guides that a rate calculation confirms or overrides.
Final Thoughts
What size aquarium chiller do you need comes down to a continuous cooling rate, not gallons or a fixed horsepower mapping. Measure total volume, set the drop, convert to energy and then a rate, add the continuous heat load, and match that rate to a class, confirming feed flow and ambient under your real conditions.
Ready to decide? Our #1 pick for 2026 is the <=10-gallon nano.
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