Water hardener for increasing GH and KH simultaneously sera GH/KH-plus simultaneously increases the carbonate and total hardness in tropic aquariums. It allows hardening R/O water or soft tap water in a directed way as well as balancing the natural hardness component consumption. Furthermore, the parameters can be adjusted for keeping hardness loving fish species. The precise adjustment of these important environmental factors improves the supply of aquarium inhabitants with essential mineral substances such as calcium and magnesium, and effectively prevents dangerous variations of the pH value by a sufficiently high KH.