Thanks for the reply. Do you reckon its harmless. It looks a bit different than your pic maybe its just younger, 5mm ish long. Its strange its the second one i found. And i've recently 2 days ago dewormed the tank to get rid of hydra that i think came in on some bolbitis. So it survived being dosed with Flubendazole
If it's Caddisfly as per Hoggie's suggestion, some of your larger fish will eat it with great pleasure.
But if you have no fish at all, I'd personally keep it and observe the transition to the terrestrial form.
No real large fish at the moment largest are forktail rainbows and rummys. I might have to buy more fish so. To late i already removed it out of frustration. With the hydra annoying me then this. I should have put it in a jar really. Just never seen anything like it or anyother pest in the 5 or so years iv kept fish