Any Idea What This Is?

[Saedcantas]

It's 100% a "glassworm" midge larvae.

As far as I know these guys are perfectly able to lay their eggs in saltwater marshes or brackish areas and therefore have a rather tolerant larval form

Your fish will eat it just the same anyway
Lotte*

 

[dusky]

Ah yessss thats the thing. many thanks all. I'll stop being nice to it now knowing its not a fish,,,,it'll be someones brekkie in morning

 

[BigC]

Just an innocent Glassworm in the end then. Still not convinced that they can survive lengthy periods in saltwater but there you go.
Well done Lotte.
Regards
BigC