A pond pump and a filter are very different from each other. But both of these pieces of equipment is necessary to keep the pond from looking like stagnant water. A pond pump is similar to other water pumps in any garden. Its function is to keep the water moving to avoid stagnation. A pump is required in a pond regardless of whether there is fish in it. A filter, on the other hand, is to keep the pond water clean and habitable for the fish. That being said, some passive filters do make use of a pond pump for the filtration process.