A garden pond filter is not necessary if a pond has goldfish only, no koi, is balanced ecologically and the goldfish are never fed.
Some garden pond owners feed their goldfish. If you feed your fish, you will need a filter for your pond. You can buy a filter for several hundred dollars or you can make your own for under £13.00. A pond biofilter hosts a colony of bacteria that biodegrades fish waste, keeps down harmful ammonia levels and keeps your pond water clean and clear.
Lava rocks are dusty and dirty. They have been sitting outside for months and need washing before you put them in your pond. Wash them using your garden hose. Do not wash the rocks except in the yard. If you wash them on a patio or in a driveway, you will have to clean the patio or the driveway afterwards.
Put the clean lava rocks in the mesh bags or pantyhose legs. Do not put too many in one bag because they can get very heavy when full of water and fish waste. Make sure you can easily pick the rocks up out of the pond.
If your grow pot is old, wash it. Put the pump, with the tubing attached to it, in the grow pot or other container. Lay it on its side for better performance. Let the tubing, which will eventually go over your waterfall or connect to your fountain, stick out of the top of your container. Make sure the electrical cord is also sticking out the top of the container.
Put the bags of lava rock on top of your pump in your container.
Place the pot with the pump and the lava rocks in the water. Plug in the pump. The pump will suck the water through the lava rocks, out the tubing at the top and be a perfect biofilter.
Give your new biofilter a jump start with a pond bacteria like Microbe-Lift PL.
Always use a Ground Fault Circuit Interrupter electrical socket. It cuts off electricity immediately if water touches it.