Size of the pond, the degree of sun and/or shade at the pond site, the amounts and types of water-plants, and the number and sizes of the animals living in the pond are all factors to take into consideration when choosing a filter. More sun exposure leads to more algae buildup; more water-plants produce more plant debris; and, of course, larger animals produce a greater bulk of waste. 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.
As a "rule of thumb" a filtration system should be large enough to filter one-half of the water in the pond in one hour. This means that, for example, a 300-gallon pond would need a system capable of filtering 150 gallons per hour. Internal (in-the-pond) filters are suitable for smaller ponds. Larger ponds require larger size and capacity pumps and large filter units to efficiently handle the greater volumes of water. Therefore, external (out-of-pond) filtration systems are normally used for larger ponds.
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. I think you need at least 1 1/2 inch pipe for a smaller pond filtration system to work efficiently; 2-inch pipe should be used to plumb larger systems. Many people will move into a house with an existing water feature, such as a fountain, and think they can go ahead and buy a filter and create a water garden without modifying the plumbing. This is just not true! Most fountain systems were not designed for future use as pond filters. Fountains do not require as large diameter piping as a pond with plants and animals does.
We keep a variety of water-plants as well as several species of fish in the demonstration water garden at Sperling's Nursery. We use a Beckett Bio-pond internal filter, "internal" meaning that the filter unit is placed in the pond itself. It is very effective against the "pea-soup" types of algae and is easy to maintain.
We chose this system because it is oil-free. Some widely-used systems use an oil-based lubricant in the pump assembly. This oil, which the manufacturer claims is non-toxic, can leak from the pump into the water, causing an oil slick on the surface. While the oil slick may deter insects such as mosquitoes from laying eggs on the water, it isn't very attractive and I wouldn't want oil all over my plants and fish. The little fish keep the mosquito larvae under control in the demonstration pond.