Saltwater systems are maintained at a constant 26-27°C (79-81°F) which emulates the natural conditions of the reefs of the world. Although our reefs can tolerate temperatures up to approximately 28°C (82.4°F), any higher sustained temperature can be detrimental to your system.
Although fish are more tolerable of higher temperatures, corals get highly stressed. They will eject their zooxanthellae, a symbiotic algae which produces the colour, and bleach. This has been seen on an unprecedented scale over the last few years in most of the coral reefs of the world due to the rising sea temperatures.
As a reef owner, you will also need to consider the consequences of evaporation brought on by higher temperatures.
If you live in a climate which experiences prolonged higher temperatures, an aquarium chiller will become a necessary requirement to successfully keep a reef tank at a constant preferred temperature.
For short term temperature increases as experienced in places like the UK, there are alternatives to the significant investment of an aquarium chiller.