These are highly specialized filters used to "polish" the water. They use Diatomaceous earth powder to coat a filter bag and filters water down to microns in size. Due to the high efficiency of these filters, they are to be used only sporadically and for short periods of time. If you have more than one aquarium a Diatom filter is a good thing to have.
Monster Diatomaceous Earth (DE) filter
Coppersafe is safe to use with UV Sterilizers, Protein Skimmers, Wet/Dry and Diatomaceous earth filters. After treatment, Coppersafe can be removed from the aquarium by water changes, freshly activated carbon or other chemical filtration resins/pads.
I am in the process of designing my sump in a 29gallon high aquarium
Filtration is the process that allows us to keep fish alive and healthy outside their natural habitat. It helps clean the water so that the fish can live for extended periods of time without constant aquarium maintenance. Waste products excreted by animals which include ammonia are extremely stressful and life threatening. The primary purpose of filtration is to remove toxic waste products produced by aquarium animals. They work by using a combination of three types of filtration:
Mechanical filtration is the removal of particulates floating in the water column. Generally, this is the first stage of filtration, and is a physical process acting like a sieve as water passes through a medium that selectively traps large particles out of the current. This can help keep the water from being cloudy or silty and make it crystal clear. The medium used can be substrate (such as gravel or rock), sponge, filter cotton floss, a micron sock or diatomaceous earth powder. Mechanical filters will clog over a time period and will need to be cleaned or replaced when this happens.
Diatomaceous Earth - Reef Central Online Community