you can run the pump, but only at full speed, to control it you need a controller either made by tunze, or one of the modules that goes with aquarium controllers.
Not true - or at least not true on my older units. There is a slider on the power cabling that allows you to vary the output, from 12v - 24v.
That is what I read in the description, but is it variable? or is a 2 way switch?
Even so maybe I'll just stick with 2 K4's or K5's, I really don't need to vary the flow...
It is a slider with multiple voltages available in I believe 4v increments from 12-24v, I would have to take apart my system to get at the cables to confirm and right now I'm not that motivated.