On Fri, 21 Jun 2019 15:19:01 +0100
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?
Perhaps there is a better way?

Just get a 6V power supply with adequate current - it will probably
waste less energy as heat than any lash-up you're likely to cobble onto
the existing one.