Given the extortionate cost of electricity and that the fact that NPower want to double the bill in their estimates I am desperate to reduce my usage. The difficult bit is reducing the submersible pump driving the filter in a koi pond. The pond is not heavily stocked so was considering switching the pump off for the winter and draining the filter down but then I will have to start the filter up in the spring. Can I easily control the speed of the pump and pump less water but keep the filter ticking over? The question is how?
It's not a good idea to vary the speed of the pump, it relies on water for cooling. The heat generated is not linearly related to the water pumped, it may overheat.
As a zero cost option, partially close off one of the water valves so reducing the flow or fit a cheap timer and run the pump part time. One of the plugin ons would be OK.
I run my koi pump only by day when the PV panels are operating
It depends on the type of motor and the design of the pump. I'm not familiar with koi pond apparatus. What's the power rating, and any other rating plate info it gives? A picture of it might make it obvious what the type of motor is. It will almost certainly be an asynchronous induction motor of some type.
If it's designed to operate with a small slip (i.e. near synchronous with mains frequency at max torque), then slowing it really needs a variable frequency drive (VFD) supply, which is not going to be cheap. Slowing it by using a simple series impedance or phase control (light dimmer) is like using a motor which isn't powerful enough to get to its designed operating speed, and you risk overheating it, whilst also not saving much energy. This type of motor is usually of cylindrical build and needs a capacitor which may be a separate smaller cylinder or a bulge in the casing of the motor.
If it's a low power shaded pole induction motor, it probably isn't operating anywhere near the synchronous frequency anyway, and slowing it down (increasing the slip) using a simple series impedance or phase control (light dimmer), within reason, won't have a bad effect on it. The larger submersible pumps I've seen are not of this type though, as shaded pole motors are too inefficient at the power ratings normally involved (50W is typical max rating for a shaded pole motor, and they can't be built for anything much over 100W as they would overheat in normal use).
As harry pointed out, when slowing a motor, you may also find it no longer provides sufficient cooling. This is an issue where forced air cooling is driven from its own shaft. If the heat conduction is directly to the water, that has a higher heat capacity it may not be such a problem.
I don't know about the OP's filter, but when I had a fish tank, turning the filter pump off was a big no-no, as it causes the concentrated debris in the filter to start drifting out back into the tank water.
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