I have a newly set up tank. ( About 10 days.) with week old Guppy fry in it. This strange thing appeared today. Did it possibly come with some plants from LFS or could it have just fallen in?
We once got a dragonfly larvae in our old planted tank, must have come in on them. Good job we removed it as it turned out they could eat fish bigger than themselves.
It's an isopod of some sort. It looks more like the terrestrial types (woodlice) than the freshwater ones I've seen, but there's no mistaking it for anything else.