Hagen is a company that produces products for animals, which some time ago, was well known among aquarists for its aquariums lamps.
Their site is probably one of the least updated in the web and is very basic.
Between its pages, however, is a small jewel: four PDF numbers of the webzine "Nutrafin Aquatic News" .
After the 4 th issue, apparently, anything was published but you really has to take a chance to read both these works and the the basic guides available elsewhere in the site ...