The key is to slightly oversize the filter in order to ensure that the pond is properly filtered. If the system fails, it's a sure indication that the filter's capacity is inadequate. This means that, for example, a 300-gallon pond would need a system capable of filtering 150 gallons per hour. Larger ponds require larger size and capacity pumps and large filter units to efficiently handle the greater volumes of water.
Balance is the key to everything when it comes to keeping a healthy pond.
Having the right filtration system, the right amount of plants and the right amount of animal life are all very important. Equally important is the right size plumbing for the pond. If the piping is too narrow flow may be restricted and the water won't be cleansed fast enough. The system will be inadequate to the job even though, in theory, the pump is the correct size. Wrong-sized plumbing is the single biggest problem my customers seem to have.