there more so an old system of filtering but when used in conjuction with a stringray / fluval they can remove particles that the internal filters can't. Thay work by sucking water from beneath the gravel thus sucking the dirt and particles into the gravel itself. The water is then pushed up through a tube with most of the dirt / particles stuck between the gravel. I tend to find that a lot of bigger particles (eg. un-eaten food) get drawn down with the under gravel system and the stingray picks up everything else.
|