Wet/dry trickle filtration, also known as a "trickle filter," is a type of biological filtration system that is commonly used in saltwater aquariums and some freshwater aquariums.
In a wet/dry trickle filter, water is pumped from the aquarium into a separate compartment, known as a "sump," that contains a bed of bio-media, such as plastic bio-balls or ceramic noodles. The water then trickles over the bio-media, where beneficial bacteria colonies form on the surface and convert harmful ammonia and nitrite to nitrate, through a process called Nitrification.
The bio-media provides a large surface area for these bacteria to colonize and grow, which greatly increases the overall efficiency of the biological filtration. The water then flows through a "skimmer" compartment where bubbles are created by a venturi pump and micro bubbles are formed, the water flow through the bubbles and the skimmer removes dissolved organics, and proteins.
Once the water has flowed through the bio-media and skimmer, it is then returned to the aquarium. Because the bio-media is in a separate compartment and is not submerged, it is exposed to air, which helps to keep the beneficial bacteria colonies healthy and active.
The main benefit of using a wet/dry filter is the increase in biological filtration and oxygenation, which helps to keep the water in the aquarium clean and clear, reduce the fish waste and other organic matter, and improve the overall health and well-being of the fish and other aquatic life in the tank.
It's worth mentioning that the wet/dry filter is not the only filtration method to use, it can be combined with other types of filtration such as chemical or mechanical filters to enhance the overall water quality, and also protein skimmers to increase the removal of dissolved organics.