right...well...gallon wise.....i have no idea. we estimated the water load to be about 50 tonnes. we have a huge Under-gravel filter that makes up the other half (unseen) of the pond. that mainly filters out the largest solids. for the other stuff like nitrates and water quality....we have a whole tonnes of water plants that help to take care of the ecosystem in there.
food wise, its Large sinking wheat-germ pellets, small hikari goldfish sinking pellets, discus tetrabits, bloodworm cubes for the "less-monster" side, 700 grams of assorted fresh food like shrimp/prawn meat with shell, fish meat and squid. Also, to complement the whole thing, we feed them a couple of handfuls of superworms and mealworms.
this happens twice a day.