It is an unstirred kalkwasser reactor. It is used in conjunction with osmolator to dispense small amounts of lime water to aquarium. In low calcium demand tanks kalkwasser known as pickling lime maintains ph, alkalinity and calcium. It is often used in large systems just to maintain pH, used with calcium reactor or two part solutions.
In nano tanks or soft coral tank it can sometimes be used as sole means of maintaining water quality between large percentage water changes. For example a osmolator and kalk dispenser used in a 20 gallon nano tank with a sump will keep sump height level for skimming and have the potential to keep up with calcium demand.
Osmolator is a great unit.The dispenser has limited applications.