Their situation and ability to share those flukes with their new tank mates is why quarantine and treatment with a deworming treatment is so important. There are quite a few wee beasties who can be cleaned out before they can do too much damage that way. And your regular aquarium isn't devastated and fry killed.

Please see the Quicklinks on Camallanus and disease prevention.

SL is correct, that scratching can be for other reasons, including polluted water, nitrite poisoning, a rapid acid drop or rapid pH rise and other external parasites and maybe lesser wee beasties. It is not just for recreational purposes though.

Not something I would leave un-responded to, for long. Start with more partial water changes. Medicate if it continues, maybe starting with a salt bath and only then using the anthelmintics and dewormers. Wild fishes, such as Apistogramma and many imported catfishes, can especially  introduce flukes to a shop or tank.