Worms in Platies?

[darkpixels]

I have 30ish platies in a 20 gallon, well heated and filtrated tank. I have found now two dead fish this year with the same symptoms, 3 months apart. They had a large pit in their stomachs, a thin, white string hanging out of their tummy, and they were lying at the bottom of their tanks. I desperately want them to be better and I have not been able to find the cause for it. Whether it is a parasite or not, if you have any idea, please let me know.
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[Matt68005]

Camallanus worms there should be red paintbrush like thread hanging out their vent.
Do you see this in any of them?
Are they all still pooping normally?

 

[darkpixels]

I haven't seen any red threads with that behavior, I have only seen white threads in the two dead fish.
Their poop is a little odd though. Instead of being a light brown and a large amount due to the fact that they are platies, they are white and there is a lot less of it.