Thanks Darrel, That does look quite a bit like it (the only difference is that it is a lot lighter in coulor) I have taken it out of the tank anyway).Hi all,
Definitely not a Dragonfly nymph, it might be a Caddis (because of the case of plant fragments), but I think it is most likely to be an aquatic moth caterpillar.
I'm pretty sure it won't bother your fish.
Something like a China-mark moth, this is the "Small China-mark", really common in ponds in the UK.
<http://ukmoths.org.uk/show.php?bf=1354>
cheers Darrel