The short answer
To cool an aquarium, increase evaporation and surface agitation: point a fan across the water surface, float sealed bottles of ice, remove the lid, and dim or raise the light. Do it gradually — around 1–2 °C per hour — because a sudden temperature crash is as harmful as the heat.
Quick fixes, most effective first
- Cooling fan. A small clip-on fan blowing over the surface evaporates water and pulls heat out, typically dropping the tank 1–3 °C. This is the go-to summer fix.
- Ice bottles. Freeze bottles of dechlorinated water and float them, checking the thermometer and removing them before you overshoot. Never tip loose ice or cold tap water straight in.
- Open the lid. Lifting a glass lid or hood lets trapped heat and moisture escape and improves gas exchange.
- Dim or raise the light. LED fixtures add heat; dimming them, raising them, or running them at night when the room is cooler all help.
Prevent it next time
If summer overheating is a yearly problem, plan ahead. Move the tank out of direct sunlight, leave a permanent gap in the lid, and keep a fan on standby. For larger or high-tech planted setups, a dedicated aquarium chiller is the reliable answer, though for most home tanks a fan is enough.
Watch the temperature the whole time with a separate thermometer, and don’t chase a target so hard that you overcool — steady is the goal.
For diagnosing the cause and safe temperature ranges, see is my aquarium too hot? and what temperature should a tropical aquarium be? If a stuck heater is behind it, check why is my heater not working? and browse aquarium heaters.