A normal setup of trickle filters has an overflow box located behind the tank to deliver water drawn from the aquarium using a pump that goes through a large box filter filled with bio-balls. The feed of water is controlled at a very slow rate and the purpose is to allow small amount water to drop to the distribution tray so that it can spread out evenly (maximize to large surface area) before moving to the compartment containing the bio-balls. Because the feed rate is generally very slow, this compartment is mostly dry except for the lower bottom few inches. The water that flows down is first collected at the sump before it goes to another compartment through a sponge block filter media before it is returned back again to the aquarium.
The diagram below will give a better illustration on what actually takes place and the whole system is actually not that difficult to setup. If you are a DIY kind of guy who prefer not to purchase a ready made filter from pet stores, you can actually construct your own system so that you can lower down on your aquarium budget and you can use the money saved from this for something else.
First due to the maximum exposure on surface area, the water loss from evaporation is very high meaning that you will need to regularly monitor the water level. This is made even more difficult especially for saltwater aquarium because this will affect and change salinity level tremendously. Secondly, compared to other types of aquarium filter, the capacity or volume of water involved in the filtration is much lower since you will need to control the feed rate in order for the system to function efficiently. Therefore this type of setup is mainly to serve mini aquariums or small fish tanks unless your resize the box filter to match your aquarium capacity.