How do I stop my tank from overheating in hot weather?

First off leave your heater alone. We see far more issues of people leaving their heater off once the weather turns cold. I’m writing this in the UK. We’re in the middle of our summer, it is often measured in days not weeks, and then something approaching winter will roar in for a few days.

So your tank temperature has risen, assuming you have tropical fish, your tank isn’t overstocked, and there are no other issues, most fish will be fine.

Firstly are the fish at the surface gasping? Or are they swimming about normally? If there’s been no change in behaviour then don’t worry, they’re coping fine, focus on keeping yourself and any other pets comfortable.

If the fish are swimming nearer the surface or in any way look uncomfortable.

Step 1. Check your heater is off, heaters sticking on is an issue at any time of the year. If the heater light is on, then turn it off and order a new one. Check the temperature, it’s far above the air temperature then do a 10% water change. If they’re still struggling then do another 10% change, repeat until they’re not struggling, or the temperature is back to where it normally is. Don’t forget a lot of aquarium lights can help warm the water, so turn them off.

Step 2. If your heater is turning off, test your water, anything other than 0 ammonia, 0 nitrites, and nitrates below 20ppm (or your normal nitrate level if you have fish that can cope with higher nitrates) is an issue. If any of those three is too high do an 80% water change and test again, repeat until the ammonia, nitrite, and nitrates are all in safe levels.

Step 3. Assuming your heater is working fine, and the water parameters are all safe, then it may be the heat impacting the fish. Is the filter outflow running across the surface of the water causing ripples? If not, can you adjust it so it does?

Step 4. Is the sun shining directly on the tank? Can you put a damp towel over the whole tank?

Step 5. Can you add an airstone to put more oxygen into the water column?

Step 6. If the fish are still gasping then you will need to start actively cooling the water. You only need to take it down a few degrees normally, can you point a fan so it blows over the surface of the tank, this will normally mean taking off the lid, so be careful if you have any fish likely to jump, they’re more likely to do so if the water is too warm. Adding bottles of frozen water can help, but they may reduce the temperature too much too quickly, but if the fish are gasping at the surface then it’s vital to save them.

Step 7. Do a water change. This will cool down the water, but the general heat will drive it back up pretty quickly. If I have to, mostly due to sensitive species, I remove 70% of the water and put the hose to drip back in filling it up over the next hour or so. Make sure you add the dechlorinator in advance.

Step 8. Consider a chiller. They’re not cheap to buy or run, but if you need to it’s a great option. I’ll be honest I’d get aircon for the room before then, but I grew up in the tropics where houses don’t bake at the slightest hint of sunshine.

Generally, the heat will stress us out more than the fish will get heat stress, but it’s worth having a plan in case they start to struggle.