The short answer
For most tropical freshwater fish, anything sustained above about 28–30 °C is getting too hot, and above 30 °C it becomes dangerous. The clearest warning sign is fish gasping at the surface, because warm water holds less oxygen. Check the temperature with a reliable thermometer before acting.
The warning signs
An overheating tank shows itself through the fish long before the glass feels warm:
- Fish hanging near the surface or crowding the filter outflow, gulping air.
- Rapid gill movement and lethargy.
- Faster, more frantic swimming early on, then sluggishness.
- A summer heatwave, a light sitting too close to the water, or a heater stuck on.
Confirm with a thermometer rather than a guess. If it reads well above your target and climbing, treat it as urgent.
How to cool it down safely
Bring the temperature down gradually — a sudden drop is as stressful as the heat itself:
- Add surface agitation. Point a small clip-on fan across the water surface; evaporation cools it and boosts oxygen. This is the single most effective fix.
- Float sealed ice bottles. Freeze bottles of dechlorinated water and float them, removing them before the tank overcools.
- Open the lid to let heat escape and gas exchange improve.
- Raise or dim the light, or switch it off during the hottest part of the day.
For the reverse problem and general temperature targets, see what temperature should a tropical aquarium be? and the related how do I cool down an aquarium? If a heater is stuck on, our heater not working checklist helps, and you can browse reliable models in aquarium heaters. Surface agitation also ties into why fish gasp at the surface.