I checked my pump, its a Hozelock Cascade 1400 which pumps 1400 litres per hour, but the recommended for the UV clarifier is Tetra Pond FP 2500 which pumps 2500 litres per hour.

So it looks like its not to do with too fast a flow. I would have thought a slower flow means more chance of UV working?

So therefore should I see the UV light as there is none showing. Its the last thing I can think of, that the bulb is not working or something.

Thanks