I'm not sure what the aquatic worms in my garden bin are but larvae love them and they look very similar to what the US call blackworm. The largest are 4cm long, dark red/purple in colour and segmented. Not thin and curly like tubifex. They're fiddly to collect in large numbers but if your search draws blanks Oceanblue I can provide you with some. It would be interesting to compare them with Caleb's. I believe mine were introduced with some water lilies about 10 years ago.
When I dig some out I'll take a video of them.