Heat wave: keeping a 60-liter tank under 28 degrees
The apartment hit 31 °C on a Tuesday afternoon and the tank followed about four hours behind. By evening the thermometer on the glass read 29.4 °C, which is the highest I have ever seen it. The heater, obviously, was off; it had been off since June. The fish were fine but breathing faster, and the cherry shrimp had all moved to the top third of the plants, which I now treat as an early warning.
I tried the obvious things in order. First the lid came off, which dropped the surface temperature about half a degree over two hours. Then a small desk fan angled across the water surface, which did most of the work: evaporation brought it down to 27.8 °C overnight and I was topping up almost a liter a day with dechlorinated water left standing in a jug. I did not use frozen bottles. I have read about it, but a 60-liter tank reacts quickly and I did not want to swing the temperature two degrees in twenty minutes and then watch it climb straight back.
Oxygen was the bigger concern than heat itself. Warm water holds less of it, and the plants were pearling less under the stronger light, so I shortened the photoperiod from eight hours to six and raised the filter outlet so it broke the surface and rippled it. The flow looks untidy but the gas exchange matters more than the view for a week.
Water tests during the three days: ammonia stayed at zero, nitrite zero, nitrate went from about 10 to about 20 mg/l, which I put down to the shortened plant day and a reduced feeding schedule that I overdid on the second evening. One 30 percent water change with water at 26 °C brought nitrate back to around 10 and the tank down to 27 °C, and the shrimp went back to the moss.
What I would do differently: start the fan on the first warm morning rather than the first hot evening, and skip feeding entirely for the hottest two days. The fish did not need it, and the uneaten flake was the only thing that moved a test result.