On Friday, 21 June 2019 15:27:57 UTC+1, Graeme wrote:
I have a 12v aquarium pump supplied by a 12v power supply, but, for my
purposes, the pump performs ideally when running at 6v. Can I just put
a resistor between the power supply and pump, to reduce the voltage, and
if so, how do I calculate which resistor I need?

You need to know the current taken by the pump.

Then Volts / Amps = R of the pump in Ohms, and place the same R in series.

Power rating of resistor Watts = Amps * Volts

Owain