wat about 5 more days and if they are doing better put him in.

I would not return it to the tank. you dont know whats been wrong, its doubtful the problem has disappeared, chances are its still infected even if symptoms not showing now. a guppy is not worth risking a whole tank of fish for. guppies are usually very hardy and excluding white spot, which obviously this isnt, they are rarely get sick.
I know it sounds harsh, but thats my opinion, ive kept fish most of my adullt life.

White string poo is an indication of an internal infection unless you’re feeding your guppy white food. You can get medicated food at your pet store to give him for this. The red marks are from when the infection spread from his intestines to the rest of his body.

Also, guppies like a little salt in their water (any kind that doesn’t have iodine added, like rock, kosher, or canning salt). About 1 tablespoon per 5 gallons. But you want to be careful if you have tetras, catfish, or other scaleless fish in the community. They can tolerate some salt, but it needs to be added slowly.

Quarantined the fish is good. However, after separating it out from the tank, did you give it medication for it to get well? That is the most important part of the quarantine. If you just separate without medicating it, what’s the point as the fish will still die however you might have saved an aquarium full of uninfected fishes. Pls give it its medication and monitor it till you see that all the symtons of the disease you’ve mentioned is gone and your fish is swimming and eating well before releasing it back to the main tank.

You should just eat the guppy. I did it too find out what was wrong with my guppies. A day after eating mine i threw up A LOT and got a fever. My doctor enjoyed my story of how i got sick. Eventually i found out my fish had sepsis or something.

I would leave him isolated for a couple of weeks before thinking about putting him in with other fish….

Dont know what it is….but it sounds bad – - and contagious.