This catch all category of pollutants means anything added to the pond that is not wanted in the pond. Pollutants can consist of items which may or may not be harmful to the fish and they may or may not be visible. They may float on the surface, sink to the bottom or dissolve into the water. They may come from outside the pond or from within the pond itself, i.e. oil leaking from a submerged pump. Some pollutants are easy to identify and control and/or remove, i.e. leaves, pollen, dead rats, etc. Some just add to the filter load if not removed but cause little other problems if not in excessive amounts, i.e. bird droppings. Most of the harmful pollutants that dissolve into the water are hard to identify or quantify. Surface water runoff that can enter the pond is often a major source of pollutants. This is why all ponds should be designed with a raised edge or at least some type of channel around it so that the surface water will not enter the pond. Any roof runoff from adjacent buildings should also be controlled. Other than preventing these pollutants from being introduced into the pond, they can only be controlled through water change out procedures.
Here in sunny southern California, where we get very little rain over long periods of time, heavy buildups of "stuff" on the covers, shade cloth, plants, or trees hanging over our ponds often occur. When it does rain, all of a sudden there is a large amount of this material that is washed off and added to the pond water with possible detrimental effects. (We are all familiar with oil coming up from the roadways during a rain following a dry spell and how the cars go slip-sliding down the interstate.) If the material overhanging the pond is rinsed off with a hose every couple of weeks, then the individual additions are much smaller and are more easily controlled through the routine water change outs. This rinse down of overhanging material should be part of each pond keeper's routine pond maintenance (at least monthly).