If you are receiving enough turn (lift) of all of your water to the surface then 5 diffusers (assuming they are sized properly) will do fine.You may be lifting 2-3 times per 24 hours. Water and bottom soil (muck) temperature is key in decomposition. example > you can be 100% saturated with DO @ 50F (11.3 ppm @ sea level) and have very little decomposition taking place compared to 6.9 ppm DO at 95F also 100 % saturated, in this case the lesser DO water will convert organics to co2 and h20 faster than the high DO water.Its all about balance. 39F winter bottom water may even be supersaturated with DO but no decomposition of organics is taking place compared to warmer temps.In your case the DO will "diffuse" to all areas. Even though often the shallow water sees very little boil action from the diffuser it will get enough natural wave action in depth to 4 ft to have its own supply of DO.Keeping water cooler for cold water types of fish and wanting decomposition of organics is also a tough balance.My best result on muck reduction has been 2 inches per year but that was up to 4 -5 turns per day, I would say 1-2 inches is more realistic even though I read claims far better than my experience.