I suppose the answers to big pond questions start with two realizations: The chemistry is the same, but the size prevents using a traditional filter. So you try to enhance and assist nature's normal processes, attack anaerobic areas, and fight the normal trend toward acid water (including the acid rain that comes in from the west). I got native catfish to stir the bottom, bringing water and oxygen into the anaerobic bottom junk, add air and microbes, and watch the pH. It even helps to fight the duckweed and algae problem. Anyway, I have not found a good book that applies familiar ideas to the larger scale, so I'm asking lots of questions.