sorry, haven't gotten back to this...
my biggest pond averages 3 feet deep, we keep all fish in over the winter. during the winter i take a small pump run it into a shallow dish (like a dish that goes under a flower pot) set it on the side at an angle so that it runs back into the pond, this will keep a hole open in the ice (all that is needed, and the heaters use a good deal of electricty) the birds use this as a bath and water source throughout the winter, also.
for filtration, i use plants mostly...... water hyacinths are the best. the big pond has a bog, water is pumped from the pond up to the bog, and the bog overflows over a big rock back to the pond.
marti