My suggestion for narrowing the stream section was to avoid stagnated areas of "swamp" land. You want to pull your worst water out, saturate it with oxygen and return it....You will get poison water devoid of oxygen in summer in the deepest portions. By pulling that water out, it gets replaced by better oxygenated water and allows fish to be able to use areas that are normally lethal. You COULD widen an area for a larger biofilter, yet given the size of the pond, I personally think it would become a net negative on overall water quality...I could be wrong on that, as it all depends....on design, plants, water dept, flow rates, shape....many things.

As for the optimum plant depths...that depends on each species of plant...will is be a marginal, shallow water, submerged....