You can get empty 2L soda bottles and fill them up with water (preferably tank water) and freeze them. Freeze as many as you can. Preferably more than 10 bottles. Believe me, when the temp gets really hot, they thaw very quick and you may end up having used all ten and the first one you used haven't even froze yet. Place one bottle at a time either in the display tank or the sump if you got one.Make sure you do not drastically lower the temp. If it is really hot, you may use two or even three bottles at a time. I prefer to place them in the sump, this way the display tank is clutter-free.