That notion is one of the most misleading, and disturbing, ideas I've seen for quite some time. The water supply is massive and is constantly refreshed via rain and moving water currents. That is hardly comparable to a few ounces of water that is rarely if ever changed. Food is not limited to a single plant, with a few supplemental flakes of food every week or two. In nature fish have access to a wide variety of plant forms, as well as insects, worms, and even other fish.
Closed systems contain a plant or two, and that is all.
Over time the fish will not have a well balanced diet. They will either slowly starve to death, or become die from disease which they are more susceptible to as the result of stress. It is no more natural for fish to live in a few ounces of water, then it is for you or I to spend our entire lives living in a closet. Water filters for pure, safe water. Bestfilters, Best Prices.