Tropical fish pond southern Spain?

[Finn2004]

So my family are planning on moving to southern Spain (Costa del sol area) and I want to have a pond outside. The pond would be 2000 to 5000 gallons and contain fish such as red tail catfish,arowanas etc. In the summer months Costa del sol is perfect for keeping these kind of fish outdoors but in the winter the temperature drops to around 48 Fahrenheit (at the lowest). It usually staying around 60 Fahrenheit in the winter months. How many months would I need a heater and any advice ?

 

[Finn2004]

Anyone?

 

[kno4te]

Looks like it at night it gets 44-66 degree lows during the entire year, winter and summer. Would keep a heater at all times and let it turn on when it needs to.