Cities often use chloramines (which contain ammonia among other things) to disinfect piping systems coming from where they treat water to send to homes. High heat and other factors like rain and humidity often causes biofilms/organics to grow inside distribution lines and cause issues with acceptable water chemistries they have to abide by for human consumption and usage. These chemicals disinfect the piping systems but the caveat is that they can also very easily wipe out your reef tank also.Could the heat wave have somehow triggered high ammonia?
Your first line of defense for reef aquariums to prevent these types of chemicals from ever being introduced is to create your own 'pure' water which is why multiple stage RO + DI is universally recommended to always be used and never use tap water for saltwater reef aquariums.
