A reef tank is a patch of the tropics living inside a British house, and for most of the year that suits everyone. Rooms are cool, temperatures are stable, and there is none of the cooling gear that reefkeepers in hotter countries take for granted. Then August arrives, the spare room hits 30 °C, and the tank that looked immaculate in May is sulking by teatime. Or it is January, a storm takes the grid down at six in the evening, and the return pump goes silent.
Neither needs to end badly. The work is done in advance, in the quiet weeks of spring and autumn, when nothing is going wrong.
Summer: the room is the problem
Most reef tanks run best between about 24 and 26 °C, with a gentle swing across the day. Sustained temperatures above 28 °C are where trouble starts: warm water holds less oxygen, corals sulk or bleach, and small problems compound quickly.
The tank itself is rarely the main source of heat. The room is. A south-facing room with the windows shut climbs well past the outdoor temperature, and British homes are built to keep warmth in. Closing a blind at midday is free and often underrated.
Cooling that works
Evaporative cooling is the cheapest and most effective tool you have. A clip-on fan blowing across the surface of the sump or display drives evaporation, and evaporation carries heat away with it. It only works if there is water to evaporate, so the auto top-up needs to be full and working before the hot spell, not during it.
- Fit a fan across the sump or display surface, aimed across the water rather than into it.
- Shift the lighting period to early morning and late evening, or shorten it by an hour or two. Corals cope with less light far better than with hot, oxygen-poor water.
- Float sealed bottles of frozen RO water in the sump if things get serious, and cool slowly. A fast drop stresses livestock more than a steady 27 °C.
- Remember that a chiller dumps its heat back into the room, and that portable air conditioning produces condensate. Plan where it drains before you need it.
If you run an external temperature controller, test the cooling circuit and the alarm before summer arrives. If you rely on the heater's own thermostat, adding a controller gives you a second layer of protection all year round.
Evaporation, salinity and top-up water
Once a fan is running, evaporation rises sharply. A reservoir that normally lasts a week can be empty in two or three days, and when it runs dry the salinity starts to climb. That is the real danger in a heatwave: not the temperature alone, but a tank quietly turning into brine while nobody is looking.
Check salinity with a calibrated refractometer during hot spells rather than assuming the top-up has coped. Keep a bigger RO reservoir than usual, mark the water level so a glance tells you what is left, and refill before bed rather than when the pump starts drawing air. Use RO water, not tap water, which brings nutrients and whatever else your supplier adds to it.
If you are away in summer, this is the part of the system most likely to fail. A neighbour who will refill a container and read out the thermometer is worth more than any gadget. Leave written instructions and a phone number.
Winter: flow first, heat second
Winter usually suits a reef tank. Rooms are cooler, evaporation slows, and the heater earns its keep. The risk is interruption: storms bring lines down, and six hours without power in January is a very different problem from six hours in June.
An unheated tank in a cold room drifts down towards room temperature, and it does so slowly. Fish and most corals handle a gradual drop into the low twenties far better than they handle stagnant, oxygen-poor water. So the order of priority is:
- Keep water moving. Surface movement matters most, because that is where gas exchange happens. A battery air pump with an airstone in the display or sump buys real time.
- Hold the heat in. Wrap the tank and sump in bubble wrap, foil-backed mats or blankets, and refit the lid, leaving a small gap for gas exchange.
- Protect the filter. Flow through the sump or canister keeps biological media alive and oxygenated. If you can only run one pump, run the return.
- Leave the lights off. A day or two of darkness bothers corals far less than it bothers you, and it saves battery for the things that matter.
Stop feeding during an outage. Fish need less food than we tend to give them, and uneaten food in a tank with reduced filtration is a problem you will still be clearing up a fortnight later. Keep the room warm, resist opening the tank every ten minutes, and only add top-up water if the level genuinely demands it.
Battery backup: decide what it protects
Most uninterruptible power supplies sold for home offices are built to run a computer for ten minutes so you can save your work. That is still useful for a reef tank, but only if you have decided in advance what it keeps running. Add up the wattage of the equipment you want to protect — usually the return pump and one wavemaker — multiply by the number of hours you want, and add a margin for conversion losses.
Three routes work well:
- A small UPS running the return pump and a wavemaker for a few hours. Simple, but the battery ages and will eventually need replacing.
- DC pumps and wavemakers with a dedicated battery input. Many modern models accept a backup battery directly, which is far more efficient than converting mains power back and forth.
- A USB air pump on a large power bank. Cheap, quiet, and enough to keep surface movement going in a small or lightly stocked tank.
Then test it. Pull the mains plug and see what actually stays running, and for how long. Anything involving mains wiring, inverters or a generator is a job for a qualified electrician, and anything near water should be on RCD protection with a drip loop in the cable.
An emergency kit worth having
Keep it in one box beside the tank, not scattered across three drawers:
- A battery or USB air pump, with spare batteries or a charged power bank.
- Bubble wrap or foil-backed insulation, cut roughly to size.
- A spare heater and a spare return pump, both tested at least once a year.
- A refractometer, a thermometer you trust, and a jug for manual top-ups.
Tape a one-page plan inside the cabinet too: which plug powers which pump, what to switch off, what to prioritise, and who to call. Add the number of a local reef club or a fellow hobbyist within driving distance. Most reefkeepers will happily lend a heater or babysit a tank for an evening, and that is often what saves a system.
Spring and autumn: your two prep weekends
In spring, before the first warm spell: clean the fan, test the auto top-up, refill the RO reservoir, check the controller alarm, and clear the space around the tank so air can move. In autumn, before the first named storm: charge the power bank, test the UPS, find the insulation, run the spare heater in a bucket of water, and update that contact number in the cabinet.
Ten minutes twice a year, done when nothing is going wrong, is the difference between a rough week and a wiped-out tank. The equipment matters less than the habit of checking it.
Photo: Maksim Goncharenok / Pexels
