For your fish that require a heat source, selections of aquarium heaters are available for you to choose from.
Things to Consider:
Setting the temperature -- Aquarium heaters act as thermostats and can be set at the appropriate temperature that you wish. The types of fish you have in your tank will determine the correct setting.
Thermometer -- Be sure to have an aquarium thermometer handy so that you can monitor the temperature of your water at all times.
Bacteria and algae -- Warmer water can provide a nice environment for certain bacteria and algae. Have a good water conditioner or bacteria killing formula handy to maintain the health of your aquarium.