Trickle filters consists of a series of trays with holes drilled it the bottoms, part filled with a choice of filter media. Usually they will be stacked above the aquarium (but can also be located beneath the aquarium) and used in conjunction with an external mechanical pre-filter whose return is sprayed into the top tray, drips through each subsequent tray and eventually returns to the tank. As the water trickles between each tray, it is exposed to the air, optimizing its oxygen intake and improving bacterial efficiency. A further advantage is that any number of trays can be added to the stack to increasing the filters capacity without the need for an increase in turnover (flow rate).

Trickle filters are extremely efficient and easily cleaned but their size and trickling noise may be an annoyance to some hobbyists.