Hi,
One of the guppies I got two weeks ago was not active or even swimming, basically hiding away on the slate for a couple of hours and in cover, the other guppies were going towards it but still no movement.
I moved the guppy out of the tank into in plastic container with the water from the tank just to monitor, for about 15 minutes but still no movement.
I placed it back in the tank but wasn't swimming and just going on it's side, I took it back out of the tank and let it R.I.P.
The other fish are fine, can't see any issues and very active aswell.
Thanks Mike.