Stringy red poop - Is it serious?

[ARGieneedshelp]

Hi! I'm having an issue with a new tetra (they get sold as live baitfish in my city, so I'm not sure what species is it, though it looks VERY similar to a Buenos Aires tetra).
I left home for an hour and when I came back I saw it had a very long and "wavy" reddish string protruding out of its anus. I assumed the worst (camallanus) and panicked, so I got the fish out with a net and placed it in an improvised quarantine container.
I wanted to take a look at the red thing under a microscope to see if it was a worm or not, but it fell off at some point and I couldn't find it again. I carefully examined the fish's underside and it has no other red strings or anything abnormal, apart from looking a little skinny. I'm not sure how much it's been eating, since it refuses to reach out for food if there's any person in the room.
The tank is cycled and has been running for about a year. Already had snails, tons of live plants and a couple of tetras of the same species. I'm sorry but I can't provide exact measurements for ammonia, nitrates, etc, since I have no way to get the tests. I've always done weekly 20% water changes and I vacuum the gravel, if that helps. The temperature is 22°C

The affected fish was added to the tank about 2 days ago, after a week of quarantine. One of the older fish in particular is hostile towards new additions, so I've seen the newbie getting chased around the tank. It schooled normally with its mates while in the quarantine tank. There were 4 fish in the main tank (counting the affected one), and there's other 3 quarantined fish (all of them look healthy and active) waiting to be added (I'm doing it one by one as a precaution). All the new fish have been acclimated via introducing water from the main tank in the quarantine one during the daily water changes, plus 20+ minutes in the bag to avoid a temperature shock. I feed Tetramin flakes to all of them (the ones that come in pink, green, orange, etc). I also gave freeze-dried tubifex to the quarantined fish about 3 days ago.


Maybe it was weird feces after all and I might have overreacted by immediately reaching out for the net, but what's done its done. How much time does it have to pass until I'm sure the fish can be added back to the main tank? Is it safe to do preventive treatments with medication? Thanks in advance.

TL;DR: saw reddish string coming out of tetra. It fell off so I'm not sure what it is but I ain't taking chances.