Now is the time to start preparing people, for people with outside tanks under the patio, shed tanks or houses with no insulation and air-con. It's good to get the temp down now in your tanks to give yourselves a little more play. Heaters should be unplugged, water changes should be made to freshen up the tanks as cleaner fresher water holds more oxygen. If you have not already got one add a air stone to ripple the water surface more for air and other gases to exchange more easily. Remove lids during the day and you can leave them off at night as well when the next day is also going to be a hot one. To stop fish jumping out i replace the glass lids with egg crate (light diffuser) cut to the same size as my lids (i only remove the front lids). This allows the tanks to cool down better at night because you want the temperature down as low as you can get it for the next day. The more hot days you have in a row the night time temp in the tank gets a little higher each time, that's why if you can get the temp down before the heat wave starts say 22 degrees, i find it gives you one extra day before the shit hits the fan. Speaking of fans, a fan blowing across the water surface can also aid in cooling also. Some people use frozen bottles of water floating in the tank, i'm not a fan of this as i believe it just stresses the fish more (unless you can put it in a sump). The fish are better of with slow gradual temperature changes up or down and not sitting next to a mini iceberg, OK that's enough typing for me good luck all and may god have mercy on our little fishes souls.