ADEQUATE WATER CIRCULATION AND MOVEMENT.
A general principle (not a rule) of about 50-200 gph per 100 gallons can work for this (A lower proportional gph for large ponds, a higher proportional gph for small ponds). This can be achieved with more than one pump and even a large aeration device (air stone). As for the air stone, I find they are excellent for vertical circulation, often better than water pumps of greater gph. A large double air pump such as a ReSun 9603, Million Air 600, Whisper 800, or Maxima with two air stones can circulate at a rate up to 500 gph. A way to check this is to place the air stone down a narrow PVC tube or aquarium lift tube, then place the top of the tube just above the water level and measure the rate the water fills up a gallon container (one gallon in 15 seconds= 240 gph).
A good water pump is the Via Aqua 2600, 3300 or 6500 hydrant pump. Both have sealed electromagnetic motors and ceramic shafts.
 GOOD FILTRATION.
I prefer pressurized pond filters such as the “Clear Stream” for their efficiency and ease of hiding in the ground. But there are many excellent filters available.
For small ponds (and even medium-large), large sponge filters are excellent bio filters and reasonable mechanical filters. They are inexpensive, easy to service, and simple to attach with either a power head pump or and air stone. There now is a specific Pond Sponge Filter called the”Hydro-Pond” there are two models; one air driven, one pump driven. They are useful up to 1500 gallons and more can be used for larger ponds or in combination with other filters.
Bio falls are also excellent pond filters. I do not recommend the old style Becket or Pond Master Submersible filters; they clog easily, are hard to service, and are not efficient filters. Multiple types of filtration are also excellent, such as a pressurized filter- bio falls combination, or a pressurized filter- sponge filter combination.
 PLANT FILTRATION (Often called Veggie Filters)
This is a very important part of pond keeping and filtration. A well planted and diversified planted pond is VERY important for clarity, pond and fish health, and even fish breeding. Any plant with a good root structure that grows fast and has the majority of their leaves above water is a good candidate to start with. I recommend water iris for their strong root structure, fast growth, great nutrient absorption, and a great place for baby fish (fry) to hide feed and grow. There are many other excellent plants as well such as bull rush. Make sure these types of plants (plants with roots in water and leaves above) are planted in an area of good, but not strong water movement. This ensures that they will do their job as plant filters.
Other plants such as lilies and hyacinth are excellent for nutrient removal too, but not at the rate of iris and similar plants. They are useful for shade, which will slow algae growth
 UV STERILIZATION.
Although not always necessary in a well planted, well shaded, well filter aquarium; they are still very useful. UVs help with algae control, disease prevention, and the Redox potential (which is important for fish health and proper filtration). For the UV to properly function in a pond, the flow rate should not exceed 20-45 gph per watt of UVC. In larger ponds with high flow rates I recommend more than one UV sterilizer, with a by-pass from the main line, connected in PARELLEL not in line together. For this reason, I also do not recommend the popular filters with built in UVs, as the flow rate is usually too high to work properly. A separate UV is always best! For more information about proper UV sterilization and how it works please visit this site; “WHY USE A UV STERILIZER”
Many times I have heard of complaints that their UV did not help with algae control, when I checked this client’s pond, I found inadequate filtration and an improperly installed UV sterilizer. Even a properly installed UV Sterilizer cannot over come poor filtration and a poorly planted pond. And many manufacturers make claims of high flow rates that are impossible for proper contact time.
 CLEANING (proper chemistry):
In a healthy pond, with a properly functioning nitrogen cycle, water changes can be minimal. I still recommend a water change of 10-20% per month on a healthy, fully functional pond. In a newer pond larger and more frequent changes may be necessary.
If your pond has a KH below 80 ppm and a pH below 7.0, I would first look to possible causes. One common one is the buildup of organic mulm on the bottom of the pond. The decomposition of this will produce nitric acids which will affect the pH and KH. Since most pond keeper have koi or goldfish, this is important to note, as both these fish do much better at a pH above 7.2 and need the calcium provided by the KH for proper osmotic function. See “Proper Calcium, Magnesium and KH in Aquariums” for more information about Calcium KH, and GH.
If a lot of leaves fall to the bottom of the pond vacuuming them or raking them out is important to prevent organic buildup.
Water changes also will help with pH and KH if your tap or well water is slightly alkaline and has a KH above 80 ppm.
A final note about KH, if your KH is below 80 ppm you will NOT have a healthy pond in the long term!
For much more, please read my vastly expanded Pond article with links to even more resources as well as pictures: A Clear Pond; Pond maintenance set-up, chemistry, filtration, predators and much much more.