Once upon a time we only had access to rather primitive forms of filtration for the marine aquarium. Undergravel filters were usually the filter of choice. This required the use of crushed coral or dolomite for the substrate, since sand would be too fine to work with the undergravel filter. These filters were excellent biological filters and did a fairly good job at mechanical filtration.
Perhaps no filter has been more closely associated with the development of a "new" style of aquarium as the wet/dry filter is to the mini-reef aquarium. Before the advent of the wet/dry filter, hobbyists had little success in keeping the more demanding coral and anemone species. Introduced in the late 1970’s, the wet/dry filter instantly increased the chance of success.
The overflow box associated with the wet/dry filter system is one of the more difficult aquarium equipment pieces to understand. While it is very difficult to visualize how it works, once you have seen one in operation, you will slap your head and say, “That’s simple!”
Don Roberts