I decided on the AquaMedic Kalk Stirrer. I have been using it on my 28g Nano Cube for about two months now with an ATO and pH control from my ReefKeeper2.
This unit has done a great job of keeping up with calcium demands of my nano reef. Levels are typically 450ppm since I started using the kalk stirrer. My calcium demand is medium right now but I have been switching to mostly SPS and frags are starting to grow. I think this unit will have no problem maintaining calcium levels in this size tank.
I have had to dose Alk supplements manually to maintain alkalinity but I may have been only having a problem with a funky test kit. I think once I get the Alk up to around 10dKH it should hold fairly well with only water change dosing required.
Using this Kalk Stirrer inline with an ATO and a peristaltic dosing pump seems to work well. I have the kalk stir rod running continuously. The evaporation rate on this tank is slow so the top-off vs. kalk dosing seems to be fairly proportioned and if anything a little heavy on the calcium. The ReefKeeper2 keeps the kalk stirrer from spiking the pH over 8.4. It is set to turn of the ATO unit (thereby turning off the kalk) at 8.4 and turn back on a 8.35. This keeps my pH steady at around 8.3 to 8.4 from day to night.
The build quality of this unit is pretty good. It seals very well with the top taking a good effort to remove/install. I did have some issues with the setup as the input line check valve was installed upside down at the factory - flipped it over for a quick fix. The instructions seem to be a translation (German?) and the wording is funky. There was some confusion about the input and output but I was able to figure out the functions.
Overall the AquaMedic Kalk Stirrer 1000 is pretty straightforward to setup and operate. The top down motor with the mechanically coupled stir rod and the bottom up water feed gives the best/easiest results of any kalkwasser dosing method (manual dosing, drip, DIY reactor/doser) I have used!