Just a couple of issues to point out with stirrers:
1. Can't stand the pump kind. This is a maintanance issue as well as an overdosing issue.
2. Magnetic bar Stirrers are good and typically do not have an air gap, but they can only take so much kalk powder or they will get sticky/mushy, and the bar won't turn. I had issues with mine like that, and also OD issues due to how much it clouds the chamber. I have had to set mine to go on twice/day, and turn off the top-off during and for the following hour after the stir.
3. The Deltec stirrer is very nice in that it is a rod based stirrer. It stirs slowly and consistently, and should not OD the tank because of that. So, you are able to run top-off water continuously. The downside is the air gap at the top. As I get more experience with stirrers, I am more in favor of the Deltec design. I am NOT a DeltecBot, but I have seen them in use and inspected the way that they run. I feel that this is probably the best design, even with the air gap.