Koi ponds are delicate environments that need excellent water quality to maintain healthy fish. Filtration is the key to good quality water, as a proper filter will clean out any dirt particles as well as eliminate harmful chemicals like ammonia, as well as provide beneficial bacteria that support biological filtration. Pond filters can be very expensive, however, and if you're serious about maintaining a healthy pond, you may want to consider constructing your own. Making your own pond filter is relatively easy if you have some free time on your hands and the proper equipment.
Drill several dozen holes in a plastic trash bin with the power drill and hole-cutting drill bit. These holes will allow water to move through the filter medium you will place in the bin.
Place one end of the PVC pipe over one of the holes on the outside of the bin, preferably near the bottom, and glue it on with the silicone adhesive. This will be where water exits the DIY filter.
Attach the other end of the PVC pipe to the input of the water pump. Adhere to the pump with the silicone as you did in Step 2. The input of the water pump is where it brings in water. On most pond water pumps, the input is horizontal, near the base of the pump, while the output is vertical, shooting water out through the top, creating a fountain effect at the water's surface.
Place some rocks at the bottom of the plastic bin. This will weigh it down when it is eventually submerged and keep it from tipping over.
Place some wiffle balls at the bottom of the plastic bin. These will provide surface area for the growth of beneficial bacteria, which convert harmful chemicals in the water to harmless chemicals.
Fill some nylon baggies with activated carbon chips and tie them shut. Layer them atop the wiffle balls in the bin. Activated carbon absorbs harmful chemicals in the water.
Layer some filter floss over the activated carbon. This will serve as a physical filter, capturing particles that pass through it.
Secure the lid on the bin and submerge both the DIY filter and water pump in the pond. Placement within the pond should not matter as long as both are fully submerged.
Plug the water pump into a power outlet and power it on. Water should begin moving through the DIY filter with clean water shooting out through the pump.
Replace the filter floss and activated carbon at least once every two weeks, depending on how many fish you have in the pond. The more fish, the more waste, and the more you will need to replace the floss and carbon. Open the DIY filter once a week and examine the filter floss. If it's turning dark brown, then it's time to rinse it out or replace it.