Our pond is about 20,000 gallons.  The deep end is 10 feet deep and the shallow end is 3 feet deep.  We use veggie filters as our filtration method. These work well for us and are low maintenance.  Water is pumped out of the pond into the bottom of the veggie filter.  The water flows up through a layer of lava rock and then overflows back into the pond.  We've constructed two waterfalls where two of the veggie filters flow back into the pond.  We use lava rock to cover about 1/3 of the height of the veggie filter.  Plants grow in the water in the top of 2/3's of the veggie filter.  Korean Sunrise is the workhorse plant in our filters.  The only thing we add to the pond is Microbe Lift PL.  It isn't a chemical, just a bunch of bacteria.  It does a great job in getting rid of the goop on the bottom of the pond.  

Main Veggie Filter:

Second Veggie Filter, Waterfall, and spitters:

Third Veggie Filter:

Aerating Pumps

Determine the Volume of your Pond

You need to know the size of your pond in gallons before you can determine the size of the filters and  pumps.  Find the volume of your pond in cubic feet.  Multiply the number of cubic feet by 7.5 gallons to get the total number of gallons in your pond.  Here are the formulas for a rectangular pond.

Volume in cubic feet = Length X Width X Average Depth

Total # of Gallons = Volume in cubic feet X 7.5 Gallons

1 cubic foot of water = 7.5 gallons

Sizing the Filter

We have about 1,000 gallons of veggie filter for our 20,000 gallon pond. Below are the formulas used to determine the size of the veggie filter and the size of the pump for the filter.  

Pond Size in gallons X 5% = Veggie Filter Size in gallons

Pond Size in gallons X 25%= GPH (gallon per hour) of water to pump through the veggie filter

For example, each of the 150 gallon veggie filters would filter a 3,000 gallon pond using a 750 GPH pump.

3,000 gallons X 5% = 150 gallons

3,000 gallons X 25% = 750 GPH through the veggie filter.