New Guppy Suddenly Pooping Stringy Poop

I've noticed that the guppy I got a few days ago is pooping what appears to be stringy poop. Apparently this isn't a good sign in the tank; I checked the tank and everything is still normal:
0 ammonia, 0 nitrates, 40 ppm...
It's oxygenated with both an airstone and the filter, but I haven't noticed any other symptoms: my guppy is still very active, and there don't appear to be white spots on his body. Could it be stress because he's still new? Otherwise, I'm not strong what's going on.

Any other tankmates with the same symptoms? If yes, it is most probably an internal parasite that may require treatment.

I had the same question. If some of your other guppies have the same issue, it’s a tank wide problem. If only the one has it, it could just be stress. Just keep an eye on him, but feed him as usual.