I use the osmolator w/ calcium dispenser on my 55g and find it to work very well. While the osmolator can be used with any size setup, the Kalk dispenser seems better suited for smaller setups. Also, although I like the simplicity and size of the calcium dispenser, you might also consider using one of the DIY kalk reactor setup with the osmolator to save a little $. The Osmolator is a bit more expensive than many of the top off setups out there but it does offer some nice features including an optical water level sensor that has been found to be a bit more reliable than the mechanical ones. There are certainly more economical ways to accomplish the task of dosing kalkwasser but you'll be hard pressed to find a system that's easier to setup and maintain. Replacement part, if ever needed, are also relatively inexpensive and readily available.