[PeterP]

I have garden pond of about 2000 litres with some gold fish, grass carp and ghost carp, all seem to be happy and healthy. I notice the pump seemed a bit slow and purchased a new rotor for it. When I removed the screw on elbow to the pump it was full of hundreds of tiny worm like creatures no more than 5mm long and about 1-2mm across and brownish in colour, they were attached at one end to the inside of the pipe the other end being free to wiggle and when they came away they wiggled in a swiming motion.
The infestation was so bad that they had almost blocked the pump outlet. on further investigation I found all the pipes to my two filters were also full of these creatures.
I have had to remove all the pipes, blasted them out with a garden hose, this removed a ball of worms from each pipe, I then imerse the pipes in Jeyes fluid to remove most of the remainder from inside. I am not making it up when I say there where thousands of them, if not more.
I have in the past found a few on the filter spray bar inside the filter but have been shocked by the numbers today and some in a pipe that has only been in use for a few months.
They do not seem to be bothering my fish and only seem to be in the pump to filter pipes and not in the pond. I am concerned that they almost cause a blockage that in the end could cause my pump to fail. The pump is quite powerful at 2000L and hour and both filters have germicidial tubes in. The pond water is crystal clear and the blanket weed killer is working.
Any ideas what they are and what I can do about them?
I know they are not red worms.