Since you have selected your tank we can move on to the rest of the required materials you'll need.
Tropical fish generally need water temperatures between 22 and 31 Degrees Celsius, so in South Africa you will most likely need to buy an Adjustable Thermostat Heater, since our winters can get quite cold.
Your Local pet store will be able to assist you in choosing the correct heater for your chosen tank size, but as a general rule of thumb you need 1W per 1 liter of water.
I generally run my tanks at 26 degrees Celsius.
You'll want to look for an adjustable Heater that includes a thermostat. The thermostat will switch the heater elements on or off depending on the current water temperature.
In large tanks it is generally a good idea to put two heaters in the tank, one on each side.
This ensures that your tanks is heated evenly.
An added bonus to this approach is that if one of jour heaters fail the other one will take over its job and give you time to replace the broken heater.