Biological Filters

If you plan on keeping more than a few fish in your pond, you will need some sort of biological filtration. The other ways of filtering will still work, but incorporating a biological filter into your filtration system will be easier. Having a biological filter will also be cheaper in the long run and will be less labor intensive.


There are many types of biological filters on the market nowadays. They start at the simple, DIY (do it yourself) homemade filter chambers and go up to the self-cleaning pressurized bead filters. Usually, the money that you pay upfront for a biological filter, the easier they are to maintain. There are many considerations to think of when purchasing a biological filter:


  • Maximum Fish Load (in order to size the filter correctly)
  • Amount of time available to clean/willingness to clean (some are messy)
  • Cost
  • Amount of time the filter takes to clean/how often it needs to be cleaned


Old Style Rock Filter


One of the simplest and oldest biological filters is a chamber filled with rocks. The nitrifying bacteria grow on the rocks and when water flows through the chamber, the nitrifying bacteria strip the toxins out of it. A good rule of thumb is to make a filter like this 1/3 the size of a pond, this will allow for adequate filtration and good stocking rates. The major downfall of an "old style" filter is that if too much solid waste builds up in the rock, the water will start to channel through the easiest way. The water will pass by where all of the filter's surface area where the nitrifying bacteria are and then toxins will start to build up in the water. The best way to clean a filter like this requires some initial construction work. These are the required materials:

  • A powerful air compressor or air pump
  • PVC pipe & fittings
  • Glue & primer
  • A saw
  • Measuring tape
  • A drill


Put the air compressor somewhere near the filter chamber. Lay the PVC pipe from the air compressor to the filter chamber. Take some other pipe and drill evenly spaced holes down the length, this is so that the compressed air has a controlled way to escape. Lay the pipe into the bottom of the filter chamber, at even intervals. Connect the pipe with the holes to the pipe coming from the pump. It should look something like this:




The filter chamber should have some sort of waste pipe at the surface. The reason for this will be explained later. Now is the time to add the rock. There are many types of rock:


Lava Rock

Lava rock is very porous, which makes it an excellent choice for growing nitrifying bacteria. Lava rock, like any other rock, is heavy. Be sure that everything is in order before the lava rock is added to the filter chamber, otherwise a fair amount of time will be spent scooping the abrasive rock out of your filter chamber. Lava rock tends to clog often, due to it's irregular shape and porous sides.



Gravel is also a good choice for a filter material. Although, it is very heavy. This type of rock works the best with the air cleaning method mentioned above. You can find this at most landscape centers.


Pea Gravel

This type of rock provides the largest amount of surface area for the nitrifying bacteria. It also works well with the air cleaning method. Pea gravel also clogs with waste the fastest, which requires more maintenance.



Now is the time to test out the cleaning process. Shut off the filter box from the pond and open the waste pipe near the surface. Turn on the air pump. It should make the water boil. If the filter chamber is full of waste, it should be churning up and flowing into the waste pipe. A filter chamber needs to be cleaned anywhere from once a month to once every 6 months depending on the fish load and size of the filter.


Wet/Dry Filter

Another type of biological filter is the wet/dry filter or trickle tower. Water is pumped up over the filter and then gets sprayed down over several chambers filled with filter media, like bio-balls. The nitrifying bacteria get plenty of oxygen while filtering the water at the same time. This is a good filtering option, but they do produce some noise. They are also very hard to hide, as some trickle towers are very tall. Wet/Dry filters require a separate mechanical filter to remove solid waste.



Pressurized Bead Filter

Pressurized bead filters are the best choice for those who do not like getting wet when it's cold and smelly from washing media. The only drawbacks are fish load capabilities and price. This type of filter is easy to use. Pressurized bead filters usually have a small plastic media inside them. This media clumps together and acts like a mechanical filter by catching all of the free floating fine particles in a pond. Pressurized bead filters have a backwash feature that automatically cleans out the filter be reversing the flow inside and agitating the beads. Pressurizes bead filters should be backwashed once every week or two, depending on the fish load in a pond.



Up flow/Down flow

Up flow biological filters are filters where the water flows up through the media. An example of this filter is the old style gravel filter mentioned above. Down flow biological filters are filters where the water flows down through the media. An example of this filter is the trickle tower.


Be wary when purchasing a filter. Look for a well known place carries well tested products. Be wary of pushy sales people, they tend to distort the facts and exaggerate a filter's ability just to make a sale. Always stick to the facts, trust your gut feeling about how knowledgeable and truthful the person selling your filter is.



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