Ammonia is created from fish waste (poo), fish urine, fish respiration, and decomposing plant matter. In order to remove ammonia, Biological Filtration must occur (the de-nitrification cycle).
This is a process where water passes through/over a material high in
surface area. Nitrosomonas bacteria colonize this surface and consume
the ammonia. They produce Nitrite as a waste product. Another
bacteria (Nitrobacter) converts the Nitrite to Nitrate (Plant Food).
These bacteria colonize on rocks, pond liner, virtually anything
that is in the water. If there is not enough area for them to colonize
you will wind up with high ammonia or nitrite levels in your water.
If this should occur, either reduce the amount of fish, or install
additional surface area for bacteria to colonize.
Methodology
Ok, so there are lots of ways to filter a pond. Not all of them work as effeciently, some require more maintenance than others, some cost more than others. I think EVERY pond should have a bottom drain and a skimmer. Those would connect to filter(s). An in-pond filter is problematic as it releases gunk back into the water when you try and clean it, so I recommend an external container, with a bottom drain for easy cleaning. One of the major shifts is filtering the water before it reaches the pump (called gravity flow). This way the pump doesnt grind up the particles (making them harder to remove).
Latest and Greatest...
The latest evolution of my DIY filter, is "the Bickal Filter". Its a simple, compact one chamber design, easy to build from off the shelf parts, in-expensive, easy to clean, and scale-able to the size of your pond (for larger ponds, make the chamber larger and add more barrels). Click here for construction information. Check out my Sketchup Model of a Koi Pond Setup
Other Options...
The designs below are filters that I have built in the past. There are also examples of other filter construction in the Consultation Section. As my knowledge and experience grows so too have my designs evolved. Should the Bickal Filter not work for your situation, mabye one of the others designs may.
This filter is for a very large pond. It has three 200 gallon passive settling chambers and one 300 gallon horizontal flow bio-filter. It is hidden competly under a 12x8 deck that cantelievers over the pond. There is a access panel for each settling chamber, and a access panel for the drain valves. The entire bio-filter chamber and pump-house can be entirely un-covered. Construction Photos
This filter is for a large pond. It is a 500 gallon upflow filter with a cone shaped bottom and designed to hold 20 cubic feet of media. Construction Photos
This filter was used for my 1200 gallon indoor pond in the basement. One chamber is a 100 gallon settling chamber, second chamber is an upflow bio-filter. Construction Photos
This filter was used for my 1200 gallon indoor pond in the basement. Water flows through a pre-filter on the top the down to the bottom of the barrel and up through bio material. Construction Photos
This filter was used for my 500 gallon indoor pond in the garage. Water flows through a drip plate at the top and showers down through media. Construction Photos