In the pond hobby industry in Europe there are a lot of filter systems fitting to your needs. They are all gravity fed and require low pressure pumps to run. They are based on the japanese style of filtering with several chambers at the same water level as the pond with mech and bio filtration chambers and a pump in the last chamber.
The drawback to these systems are lack of head for fine mechanical filtration. You could reverse one of these systems however by pumping the water in first. By giving up a minimum of water head, say 5 inches, you could build up a slight pressure gradient that would be enough to run fine filters with. The water level in the first filter chamber would be about one inch above the tank water level. You just need to install your mechanical filter media in this first chamber so that when it starts getting clogged the water level can rise causing a slight back pressure.
As a safety measure the water can overflow into the next chamber once the mechanical filter causes a rise of 4 inches of back pressure because it doesn´t let enough water through. The obvious drawback is that you have to keep checking the water level in the mechanical filter chamber to check whether the mech filter needs cleaning. By installing a high surface area mechanical filter you could get around the problem of the fine filtration clogging up too quickly but cleaning day will eventually come. A variation is to use filter floss and wash or replace it (but that will depend on your budget).
I´ve recently developed a high surface mechanical filter for such a scenario which is 28 inches long, 20 inches wide and 24 inches high. This mechanical filter is meant to be submersible in a sump or the above mentioned filter chamber. It comprises 10 removable elements each built to hold a surface area 24 inch by 24 inch of filter floss (about 4 sq yards in total). The filter floss is sandwiched between two pieces of plastic gridding so that you just have to take an element out and clean it with a garden hose or replace the floss when necessary. Of course for tanks above 3000-4000gal this system starts becomming impractical due to the high mechanical loading of really large tanks.
As for the efficiency of the pump there are more efficient pumps but you could go with the Hagen max flo 2900gph with a max. head of 14 feet, 9 inch and 130 watts power consumption. This pump isn´t the most efficient you can use but at least beats the Docs´ pump for efficiency (he forgot to check the catalogues from this decade

).
As for heat loss in such a system the way the pond keepers in Germany have been doing it in winter the last few years is to cover the ponds with an insulating clear plastic sheet hung across wires suspended above the pond. Whether this system is applicable to monster tanks depends on where the tank is I guess.
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