A Mystery Solved!

Aha! Those tiny silvery slivers in my pond aren't fish at all - they're chaoborus, of course! Close up they look like the beastie in the picture in the link, but in the darkness of the pond with their silvery shimmer and bulging eyes (really air sacs) at the front, and their horizontal fish-like swimming movements and shoaling habits, I was becoming convinced they were tiny fish. At 2cm long and from the pond surface they really don't look that different to a baby stickleback seen from above (except perhaps for the pointy nose), but they look waaay different from the side - a view I couldn't get in the pond.

They are commonly known as glassworms, and are the larvae of the non-biting phantom midge (scroll down in the link for more pics). Looks like I've identified the things that looked like little beansprouts with their tails hanging down too; it's the glassworms in their pupal stage.

Well, you learn something new every day. I also stumbled across this website which (although in German) has some really amazing close-up pictures. You wouldn't believe such a tiny organism could look quite so spectacular.