Part of our Education Hub. Pair this guide with the Chiller Size Calculator (in the Marine & Reef section) – free for account holders – to get a recommended size for your exact setup.
Why temperature is the big one in Queensland
Most of the marine fish, corals and inverts we sell come from reef waters that sit between 24 and 27 °C all year. A Brisbane room without air-conditioning can spend days in the low-to-mid 30s across summer, and an aquarium slowly tracks its room – then lights and pumps push it higher again. Warm water also holds less dissolved oxygen, right when your animals’ metabolism and oxygen demand are at their highest. That combination – not winter cold – is the main temperature risk for marine tanks in South-East Queensland.
What temperature should a marine tank be?
Aim for a stable point between 24 and 27 °C – most reef keepers settle on 25–26 °C. Brief afternoon peaks of 28–29 °C are survivable for many fish, but water that sits above about 29–30 °C is where trouble compounds quickly: corals brown out or bleach, oxygen falls, and nuisance algae and bacteria take off. Stability matters as much as the number – a tank swinging three or four degrees every day is harder on livestock than one held steady slightly high.
Where the heat comes from
- The room itself. Water settles toward air temperature. Every degree your room sits above your target has to be pumped back out of the water somehow.
- Lighting. The biggest equipment source in most reef tanks. Metal halide dumps far more heat into the water than T5, which runs warmer than LED.
- Pumps, powerheads and skimmers. Almost every watt a submerged pump draws ends up in the water as heat – 24 hours a day, not just when the lights are on.
Construction changes how fast heat moves too: acrylic insulates better than glass, and a covered tank exchanges less heat with the room than an open top.
Signs your tank is running too hot
- The thermometer creeps past 28–29 °C by late afternoon, peaking with the lights
- Fish breathing fast or hanging near the surface (falling oxygen)
- Corals looking pale or brown, or staying closed through summer
- Sudden algae or cyanobacteria blooms in the warm months
A cheap max/min or logging thermometer is the best ten dollars you’ll spend – it tells you what the tank does at 3 pm on a hot day while you’re at work, which is the number that actually matters.
Before you buy: alternatives worth trying
- Air-condition the room. If the aircon already runs through summer, your tank may never need more. This is the most effective option per dollar if it suits your household.
- Evaporative cooling fans. A clip-on fan across the water surface typically buys 1–2 °C. The catches: it works least well in exactly the humid Brisbane weather when you need it, and it dramatically increases evaporation – so expect heavy top-offs and use an auto top-off to keep salinity stable.
- Switch to LED lighting. Replacing halides or T5 removes a large slice of the heat load at the source.
- Ventilate the cabinet. Sumps and return pumps boxed into a sealed cabinet cook themselves. Vents or a quiet PC-style fan help more than you’d expect.
- Shift the light period. Running your main lights into the evening keeps the lighting peak from stacking on top of the afternoon room peak.
When a chiller is the right call
- Rooms that reach 32 °C or more and can’t be air-conditioned reliably
- Reef tanks with SPS or other heat-sensitive, high-value stock
- Anyone away from home across summer – a chiller holds the line unattended
- Smaller tanks: less water swings faster, both up and down
- Coldwater and temperate species – at Queensland room temperatures these setups simply don’t work without one
Sizing your chiller
Chiller capacity is about matching your real heat load: total water volume (tank plus sump), how far you need to pull the temperature down from your room’s summer peak, how well the tank holds cold (glass vs acrylic, open vs covered), and the wattage your lighting and pumps put into the water. Our Chiller Size Calculator runs those numbers for your exact setup and adds a 25% safety margin.
Two things to know when comparing models: “HP” ratings are nominal and vary between brands, so compare the manufacturer’s pull-down chart (litres vs degrees of pull-down) rather than the badge; and always size against your worst summer afternoon, not an average day.
The undersizing trap
An undersized chiller looks fine in May and fails you in January. It runs flat out through the hottest hours, still loses ground, cycles endlessly, chews through power and wears its compressor out early – and the day it finally can’t hold on is by definition the most dangerous day of the year for your tank. If your numbers land between two sizes, take the bigger one; the larger unit also cycles less, which means less wear and a longer life.
Installing and running it right
- Give it air. A chiller doesn’t destroy heat – it moves it from the water into the room. It needs clearance and fresh airflow; sealed inside a cabinet it ends up fighting its own exhaust.
- Plumb it properly. Feed it from your return line or a dedicated pump, keeping flow within the manufacturer’s minimum and maximum – too little flow ices the heat exchanger, too much cuts efficiency.
- Set a sensible differential. Half a degree to one degree of hysteresis stops the unit short-cycling. A separate temperature controller is cheap insurance against a stuck sensor.
- Keep the intake clean. Dust and salt creep choke the condenser – a quick brush-off every month or two keeps capacity where it should be.
- Expect the room to warm slightly. Site the chiller where that heat can escape, not in the smallest closed room in the house.
Not sure which way to go for your tank? Run your numbers through the calculator, or get in touch – we’re happy to size it with you before you spend a dollar.