I wonder if they have been scared - perhaps a heron?

That does sound like they have been frightend as Linda says, and a split fin or wound like a sore `can result if they bash somthing in thier flight, keep a very close eye on that sore steve, at this time of year it can get worse quickly, if it dosent look better in a few days then treat it yourself, wounds on a petcal are difficult to heal because they are always in motion, so a bit more time is needed.

Thank you very very much. The fish all seem happy today but are still huddled. A heron was spotted yesterday so this could be so.

any change steve? jo o/

No changes at all. The fish all seem to be in hiding and only come out for food as and when they wish. The heron has not been sighted since Sunday, and if I am totally honest, I doubt it is the culprit. My pond is only small and probably cannot be seen from above due to the tree overhanging it. The nature pond is at the bottom of the garden but it is too small by far for a heron to attack.

Could the fish be spawning??

Steve

We had a visit from a heron last year and the fish were very timid for a long time afterwards.

Our pond is not very big but I think herons have very good eyesight.

when our goldies spawned (when we had goldies) it was quite a "boysterous" affair, lots of chasing, splashing, jockying for position. from what i've heard koi behave in a similar fashion only the splashes are bigger! i've heard of fish being injured in the spawning melee so it doesnt sound like thats your solution. if all params are spot on all you can do is watch (carefully) and wait i guess? ours went a bit timid recently with the thunderstorms we had but soon returned to normal. no flicking/flashing going on?
regards jo o/

Yes Jo it is exactly the same only wilder as you say, koi can and do get a bit batterd during spwaning, some get damaged too.

Steve dont think for one minute that a tree will stop a heron seeing your pond, they can see water and fish even in the dark!

Thanks for info folks. They were a little bit mental last week, flashing and following each other around. Then they all went quiet.