What’s best way to avoid frozen fish?

What should I do if my fish pond freezes over this winter? – Iain, Blackheath DAVID: Protect your pond from freezing in the first place with a tennis ball. Float the ball on the surface – when it freezes over, remove the ball and you will have a hole to let air into the pond. If it does freeze completely, use boiling water to melt a hole. Smashing the ice can cause shock waves that are harmful to fish.