PlanetCatfish.com

Hello! We have a problem with our Synodontis.
He has this white string hanging out his back end.
We had this once before and that one died shortly after.
We put some new fish with them Like 3 months ago sadly without quarantine.
The Synodontis are over 10~15 years old If it Matters.
It doesn't seem to be poop. We looked at the dead one afterwards and the white thing was stretchy? Kinda like a rubber band, but there is nothing like that in the tank which they could have maybe eaten or something.
(Photos are taken while feeding today)

Sorry, wow, this looks like a big worm. Whether or not it is, it's a great reminder of the virtue of QT.

I'd hit it with a dewormer. You can use sheep or dog dewormer (fenbendazole) if you don't have a good fish product.

Good luck,
Eric

Agree with Eric. Praziquantel is too a common dewormer. Fenbendazole is good for tough cases or where a parasite had developed an immunity to prazi and other more commonly, older-generation dewormers.