Trickle filters are biological filters that are installed underneath the tank. They are made up of a sump, which is basically an aquarium or other water container containing a biological filter medium, commonly bioballs, through which the water trickles. Water is gravity fed into the filter and dispersed over the bioballs, which are often kept in a sump partition by a spray bar or drip tray, before being pumped back into the tank from the sump. There is no oxygen constraint since the bioballs are normally at least partly exposed to air, and biological nitrification (the conversion of harmful ammonia and nitrite into nitrate) is quite effective. Additional partitions may be added to trickle filters to hold mechanical and/or chemical filter material.
The disadvantages of trickling filters include their big size, expensive cost, and the fact that they must be installed under the aquarium. A trickling filter is normally custom-made to fit the aquarium it will be used in, as well as the cabinet it will be stored in, if relevant. They need a fair amount of care since they are prone to evaporation, and if the water level is not maintained, the return pump may run dry. Furthermore, evaporation must be adjusted when utilized on marine tanks to minimize variations in salinity.
Another factor to consider is how to get water from the tank into the trickling, preferably before the tank and/or cabinet are completed. There are two approaches to this: To begin, a hole in the tank may be bored and a bulkhead fitting or a water sleeve employed. This is normally done at the time the aquarium is built. Second, a self-starting hang-on syphon may be employed. When there is a power outage or the pump is switched off for maintenance, they automatically stop and resume.
Although previously heralded as innovative, trickling filters are heavy and rather costly (3 to 4 times the cost of canister filters) and have since been surpassed in many applications by current canisters for household use. Trickle filters, on the other hand, are often used by retailers, breeders, exporters, and professional hobbyists since they can be set up to filter more than one tank, making them far more cost-effective.
They are sometimes used in big marine tanks because they enable protein skimmers to be stored in the sump rather than in the aquarium. However, the very effective nitrification given by a trickle is not always desired, for example, in reef aquariums where corals are the primary emphasis. While a trickle removes ammonia and nitrite fast, it turns them into nitrate, which is harmful to corals. Many current reef-keeping techniques forego trickling filters (and occasionally all types of classical biological filtration, particularly when no or few fish are maintained) in favor of protein skimming and natural filtering by organisms found on live rock and sand. However, a sump without biological material is still utilized to hold the skimmer and other equipment on occasion.
Making the best use of this filter system:
If you pick trickle filtration, make sure your cabinet is big enough to accommodate the filter, and think about how you will get the water out of the tank and into the filter. Choose a pump that is big enough to return water to the aquarium, taking into consideration the vertical distance from the sump to the top of the aquarium as well as the needed turn-over.
The spray bar or drip tray should be serviced on a regular basis, and any prefilters should be cleaned.