Large initial capital layout. Is it really worth it?
You need warmer temperatures when the sun is not shining anyway. You can have battery banks with an invertor and heater, but it is expensive.
A solar geyser is an option, but once again, at R 6000.00, is it really an option?
A heat pump will be more economical.
A cheap option is to run your water through irrigation pipes on the roof, but you need a strong pump and will loose water flow. Once again, it will increase the temperature when you do not really need it. However, it is an option if your pond is in the shade.
The temperature in SA is never so cold that it justifies a heater.
Rather spend the money to increase your depth.