Bio filter provides extra biological surface area for nitrifying bacteria to convert ammonia into nitrite and then nitrate. Trickle filter works by dropping water from fish tank through some kind of trickler (see A in diagram above) onto bio media (see B) and bacteria living on these bio media will help the nitrification process. Trickle filter also helps in degassing process, when the water trickle down, it contact with the air and some gas inside the water will be released to the air, such as carbon dioxide and excess nitrogen.
I use colander to create the trickle effect. This colander will be covered with some cloth, which function to filter out fine solid that escape the radial flow filter. Bio media that I use is K1 bio media, however I only have 1kg of them, which occupies about 6 liters of volume, so I added together some clay pebbles I have around to increase the bio media volume. Both media are wrapped inside a mesh cloth.