Just recently, I changed my filter media from scoria to bio balls. It is far more expensive but far more superior in performance. The only advantage using scoria/lava rock is cheap cheap cheap, but maintaining the filter is a nightmare. It is also the usual caused of chronic illness especially the fry. The materials I used are 20 liter bucket, 20 mm tank connector, plastic mess with 2 cm hole, plastic tie wraps and 15 liters of high quality bio balls. I made a cylindrical shape basket for the balls. The diameter of the basket is 4 cm smaller than the bucket. This will create air circulation within the bio filter itself. The height of the basket is 30 cm.
Application for this filter: My circular tank has 2000 liters of water in it. Power head pump of 1300 liter/hour is put in the middle of the tank. Pipe (20 mm dia) is 95 cm high that goes to my 10 liter bucket. This bucket was drilled on the bottom which will serve as a drip system. I have several open cell foam in it to filter the waste of my fish. Then the water drips below the bio filter then to the pond via spay bar effect to increase oxygen in the water. My filtration system is proven for many years now. Even the load of the pond is massive. The filter can handle 300 1 year old oranda goldfish. The more the load is, the more you clean the foam. Cleaning the bio balls is so easy now compare to the lava rock I am using before. I have 40 liter bucket filled with pond water, dip and shake the basket and that's it less than 1 minute of cleaning.
Step by step procedure by photos:
