Evilmonki

cut a square of polystyrene about half to three quarters the size of the outlet chamber. It works as a float, stopping the gurgling noise.

The Hydor heaters had serious issues with them. I am surprised if shops are still trying to sell stock of them. I know of a fully planted discus tank which got cooked by one of these. The problem is when it fails it over heats, so unless you are there watching your thermometer the whole time it's unlikely you will catch it. Not worth the risk.