Wet/dry filters are also known as trickle filters. They use a pump to remove water from the tank and then pass it down a tower. The tower is filled with material such as bioballs for biological filtration. The filter material is continuously exposed to water and air, thus increasing the capacity for helpful bacteria to form in well oxygenated water. This explains the term wet/dry.
The wet/dry filter is similar to a power filter but allows the water to flow completely down past the
filter while still exposing the filter element to air. It is also larger and more powerful than a power filter. The wet/dry filter was the most common type of filter used in saltwater tanks until the protein skimmer began to take over. They are somewhat risky, in that decaying material can be trapped in the filter and increase nitrate levels, which is why the protein skimmer has become the standard in saltwater tanks. It is appropriate to use both simultaneously in a saltwater tank. Wet/dry filters can also have chemical and mechanical filtration devices attached to them. Wet/dry filters require very little maintenance, but, as with the power filter, evaporation is a problem. There are numerous varieties of wet/dry filters and it is common for aquarists to build their own version.