Fortunately, Durban is not so cold... could have been a lot worse if you lived in Johannesburg or Bloemfontein
What is your water temperature at the moment?
I would suggest heating up small quantities of tank water in a glass/ceramic container, and then adding it SLOWLY to your sump, raising the temp back up to normal over a few hours.
Also, insulate you tank by covering the glass with styrofoam or a few layers of newspaper, and perhaps cover everything with a blanket. Putting on your lights will also help, especially if you have MH.
Good luck.
Hennie