How to cool your aquarium in a heatwave


Unless you’ve accidentally locked yourself in the fridge, you’ll be unable to miss just how sweltering it is this summer, writes Nathan Hill. Here are a few ways to keep your fish happy over this seasonally hot spell!

If you’re anything like me, you’re probably looking at your aquaria every five minutes right now and hoping that reading on your thermometer has magically dropped a little. Any just like me, you’re probably worried that it’s not.

Overheating for fish is a big problem. It’s a problem for fish metabolisms (which speed up), and that’s a problem for water quality because fast metabolisms mean more excreted waste.

It gets worse. Higher temperatures also mean lower levels of dissolved oxygen in water. One of the biggest guzzlers of oxygen in your aquarium is the bacteria colony converting fish waste. As the mercury rises, bacterial oxygen consumption goes through the roof, to the point that the bacteria are competing with the fish for what’s available — in fact, the nitrifying bacteria keeping your tank safe from ammonia and nitrite are at their most efficient between 25 and 30°C.

Get warm enough and biological enzymes start to break down altogether, bacteria begin to die or turn dormant, and you’re in a world of pain.

Is there a critical temperature? There is, but it’ll be on a species by species basis. In a community tank, your Neon tetra will begin to struggle and die before your Rams do. But there isn’t a ‘magic ceiling’ that if you stay underneath, everything will be fine. There are just too many variables. Sorry.

How do I keep my tank cool?

There’s a handful of things you can do, some more laborious than others.

Get lights off to reduce heat

Get lights off to reduce heat.

Turn the lights off!

Your aquarium lights belch out heat. Even low wattage LEDs are emitting warmth as they shine — you can’t get light without heat. T5 fluorescent tubes and bulbs churn out considerably more again.

Turning off the lights will slow rising temperatures, but take note — in a heavily planted aquarium you will want to increase the oxygen levels in the tank with an airstone or increased surface movement. Remember, warm water holds less oxygen, and in the dark your plants go from photosynthesis to respiration. They actively start to remove oxygen from the water.

Don’t touch that thermostat

Don’t touch that thermostat.

Don’t touch your heater.

Your heater is thermostatically controlled. If it’s already warmer than the thermostat setting then all you're doing is introducing an element of danger to the equation. The heater isn’t going to turn on during the hot spell, but if you don’t remember to turn it back on when things start to cool again, your fish will be in for a cold shock.

Yeah, I wouldn’t

Yeah, I wouldn’t.

Add a frozen bottle (or don’t).

This one is to be reserved for absolute emergencies, but be aware it’s pretty much as dangerous as just having an overheating tank to begin with. Get a bottle of water and freeze it. When things are getting too hot, add it (sealed tight) to the tank. Then guard it like a lion to make sure things like Ancistrus don’t come up and suck on to it.

A frozen bottle will introduce a patch of localised cold to your tank. If you have poor flow, all you end up with is one hot end and one cold end. And fluctuating temperatures are highly dangerous to fish. Immediately around the bottle, things will be incredibly chilly, and any fish that gets near will likely experience a severe shock. Hell, they even use the ‘chilling’ approach to euthanising some (small) fish in laboratories. That’s how dangerous this one is.

If you’re going to do this, do it in a tank with ample flow. And definitely don’t just drop in ice cubes of water, unless you’ve made them from your usual supply you use for waterchanges.

As a variation on this, I have been known to stick cobbles in the fridge for a few hours, and when things are warming up, I’ll push them deep down into the substrate of my tank. The idea is that the cold will dissipate through the substrate and slowly cool from below. It has a negligible effect on temperature, but it’s one heck of a placebo for me.

Use it safely and it can help

Use it safely and it can help.

Get a fan.

Open the lid of the tank, get either a dedicated aquarium cooling fan or safely position a domestic fan (you really don’t want anything mains-powered dangling precariously over your tank) and let it blow. The moving air will carry away heat from the surface, but note that at the same time it’ll increase evaporation like crazy. If you need to prepare your water for top-ups in advance, make some up ready because you could be losing a centimetre or two a day.

Keep waterchanges sensible

Keep waterchanges sensible.

Perform a waterchange.

Of course, this only works when the replacement water is cooler than the aquarium water, and it’s a lot of work for a very short-term solution, but if you’re all out of options and your fish are struggling, change some water.

Be careful to avoid two simple mistakes. First, don’t change too much water. If you usually change 25% at a time, only do 25% now. The temptation to do more is strong if it’ll drop the temperature, but the chances are you’ll alter water chemistry and risk upsetting the biological function of your filter.

Secondly, don’t add water that’s too cold, especially if you’re using treated cold tapwater. Remember we talked about dissolved oxygen earlier? Your chilly tapwater will be saturated with it by contrast to your hot tank. If you up-end a bucket of that cold, O2 rich water into your tank, it’ll rapidly warm and the oxygen will form tiny bubbles.

If that happens while your fish have also taken on a load of that chilly water (through ingestion and via the gills) then those little bubbles will form in their blood, leading to embolisms.

At most, aim to drop the aquarium just one or two degrees centrigrade through a waterchange.

Check any impellers you have running

Check any impellers you have running.

Check your pumps.

Trying the above and still finding your temperatures are racing up? Inefficient pumps could be the cause.

A dirty impeller causes increased friction against the impeller well — that little spinning magnet driving all of your filters. The more friction, the more heat, and before you know it your internal canister or powerhead might be a little 40°C radiator in the tank.

Open them up, get them cleaned, get them running smoothly.

You may need a DIY store for these

You may need a DIY store for these.

Stick some polystyrene around the tank.

Insulation cuts both ways, you know? As well as trapping heat inside the tank, a good layer of polystyrene helps to keep cold in and warmth out. Ever ordered frozen goods and had them turn up in a polybox? Same thing.

Good old fashioned ceiling tiles, some Sellotape and a bit of fumbling will help you got out heat ingress from the front, back and sides of your tank. To an extent you can do the top as well, but leave plenty of open space for gas exchange. Remember, it’s hot, oxygen is low, and you want all the gas exchange at the surface as you can get. Speaking of which…

Increased surface movement will help

Increased surface movement will help.

Fire up an airstone.

Airstones might be old hat for most, but they serve a vital purpose. Ignore recent scaremongering that air pumps and airstones can cause embolisms — they don’t. You might just be able to manage to cause embolisms if you went out of your way (if you had a marine tank, running excessive wooden airstones, with bubbles being taken and ‘chopped’ by other impeller-based pumps in the tank) but in a community with a Fluval 3 filter running, you’ve no chance.

Airstones will do a couple of things. Essentially, they’ll increase surface area, and what that does is increase the rate at which both heat and gases can exchange at the interface. It’s a double win, more oxygen goes in, more heat comes out.

Look out for impending storms

Look out for impending storms.

Watch the weather forecasts.

This is especially important if you happen to have a heavily planted tank.

You’re looking for sudden snaps of low air pressure (when a storm is due), because these will further lower the amount of oxygen available in your tank. After an exceptionally hot day, a night of low air pressure can be the final ingredient on a perfect storm (literally) of disaster for you.

In this situation, you’re up against already low oxygen levels, high metabolisms, oxygen-hungry bacteria and fish, and respiring plants. Now add a sudden extra depletion in oxygen and you could wake up to a wiped out tank.

Get those airstones bubbling at the first hint of a heatwave storm.

Reduce sunlight where you can

Reduce sunlight where you can.

Close the curtains.

Did you know that fish can get sunburn? It happens to pond fish that don’t have ample cover all the time.

Same thing in your aquarium. A big laserbeam of sunlight cleaving through your window and into the tank isn’t going to do any favours to fish temperatures. What’s more, it’ll heat up gravel, décor and equipment which will then be sat there radiating heat all afternoon.

Get your tank in the shade, and if you can’t close the curtains for any reason, make up a sunbreak from some sheets and take the pressure off of that tank.

Does it look pretty? No. But does it make a huge difference? Also no

Does it look pretty? No. But does it make a huge difference? Also no.

Invest in or make a chiller.

I mean, in the first instance, if you’re loaded, then yes. A chiller will set you back hundreds of pounds, but it’ll do the job better than anything else we’ve looked at here.

Can you make one yourself? Sorta/kinda, but it won’t be great. I once knew a guy who, in his desperation, got a cheap mini fridge, coiled a load of 12mm piping around inside it, and fed it with a pump from the aquarium, slowly, with some chilled water trickling back into his tank.

Did it work? Not really, it was pretty crap. I can’t picture it being too efficient either, based on how he botched the door together with bubblewrap and gaffer tape.

To an extent, you can do something similar if you have an external canister and plenty of aquarium hosing. Get a bucket of ice-cold water, and coil a load of excess hosing in that bucket before it goes back up to the tank. Watch the temperature change like a hawk to make sure you’re not overdoing it and dropping temperatures too quickly, though.