[concept]

Whack 2 of these in:

http://www.seapets.co.uk/products/pond-supplies/pond-equipment/pond-lighting-and-electrics/ice-control/aquael-pond-heater-300w.html?ref=googlebase

Done.

(If you keep fish, you can pop them in too)

 

[Crystal-clear]

i have a 500 liter tank you think that will stop it from freezing? how does it work have you got one?

[clearlyclean]

It leaves a hole for the fish to breath doesnt stop freezes you might as well put a tennis ball in your tank

[[GQC] Tim]

Even if it would stop freezing it's not going to stop your pipes from freezing.

By far the best thing I found is a fan heater. They sell them at Screwfix. With thermostat. Set it and forget it and open the doors to a warm van in the morning.