Your thermometer should be as far away from your heater as possible in order to get an accurate reading.
Ice can provide temporary cooling for heat waves or sudden temperature spikes. Use treated water, not just any ice, though! Using an aquarium chiller is a permanent solution ideal for tanks that are always too hot or for saltwater tanks.
Direct sunlight cannot replace a heater and can cause algae blooms.
You can place the filter cartridge in treated water in order to avoid harming the beneficial bacteria there.
You might want to let the water settle before adding fish.
If your tank has live plants and a light, consider leaving the light on for fewer hours per day during hot times of the year. All lights generate some heat, so less light may help the water temperature. If the lights are inside the tank’s lid, remove the lid if possible while the lights are off in order to improve airflow. If your tank does not have live plants, it does not need a light. Keep the aquarium lights turned off. Also keep the lid off the tank if possible to improve airflow and further cool the water.
LEDs are a popular and good choice. Avoid fluorescent or halide lights, as they grow hot quickly and can even pop because of the heat and moisture.
You can also buy heaters that automatically turn themselves off at a certain temperature.
Drop-in chillers are simply dropped into the tank, commonly right in the sump or directly in the outflow water current so cool water will be circulated. It’s easy to install, but also easy to get wrong. Some people also warn against putting a chiller in your filter line as it can kill beneficial bacteria colonies leaving fish vulnerable. An inline chiller requires more setup and often is more expensive, but is less noticeable than a chiller inside of a tank with less risk to your bacterial colonies. The chiller sits outside of the tank and has lines that run into the tank. It sucks up warm water and expels cooler water via these lines.
