The RJ cable carries the power so it would be easiest to power it through the cable like it was intended and power off of the 12v going to your reef pi and Darlington transistor. I suppose you could open it up and attach 12v directly to it but this would require etching the motherboard inside.The doser can be powered externally, correct? I suppose I would just need signalling to go to the motors in the housing. I think I can figure that out.....