Chip wrote:

Perhaps a radical idea- Could you hinge the grid (either on one side or
both sides to open in the middle)?

I _hope_ that's not radical - I just assumed it would be the case. It's
really necessary that the grid be easily removable.

Even more radical- Could you use the grid itself for heating? Perhaps
with a low voltage source.

Ack! This was supposed to be a safety feature :-) Really, low voltage
means either low power or high current - the first restricts your heating
ability and the latter means huge cables (or a transformer at the pond).
Then you have the fact that your heating source would be at the surface of
the pond - and radiating most of its heat into the atmosphere.
--
derek