Heated tank for sharks

Question
I recently received an aquarium with sharks as a gift.  It has 3 different types of sharks.  Bala, silver tips and red tailed.  My question is should I heat my tank.  I had 2 sharks dead in my tank this am for no apparent reason.  It is not cold in my house except cooler at night but probably around 65 or so.  Thanks for your help.  Sharks are a new fish for me.

Answer
Short answer is you need a heater, the 65 is too cold.
The temp should be in the 76-80 range.
I would get a good submersible heater allowing at least 3 watts per gallon of tank, ie, 30 gal tank, 100watt heater.
That would be the minimum heater for that tank, you can always go larger as well.
If you have further questions, just ask