Weird thing attached to fish.

Hey. 3 days ago we brought home 10 mountain minnows. They had all schooled up and was swimming great together about 10 minutes after adding them to our tank. We sat and watched them and we noticed something White hanging from the fish.
We took a closer look and it looks like its hanging from the fishes gills or mouth. It was still swimming perfectly, eating and schooling.
3 days later... today the fish isolated itself at the top of the tank the white string thing still hanging from it. Also its fins aren't looking like the other 9 in the tank. Its top fin almost looks missing.
We caught it in the net and tried to remove it but the fish moves with it and struggles as we touch it. We added general cure and melafix the day we noticed it(3days ago) but its still there.

We also posted and asked what it could be and people said possibly string from the net and the guy in the fish shop said it also looked like a piece of fibre.
We are worried as our fish doesn't act or look healthy compared to how it was 3 days ago.

What is going on with our fish and what might the white string be?.

 

I’m pretty sure it’s some sort of parasite but I don’t know what kind I don’t think gill flukes because those are smaller

 

I thought that too. But its really long and curly. Doesn't look like an anchor worm at the end either. All the other fish are doing great too. Just this one

 

What are your parameters? And did you cycle your tank?
Is the most recent picture of another fish having it? I would quarantine any fish with it, just to be safe and hopefully not spread it.

 

Yes we cycled it before adding fish.
Our tank has been set up for 2 months.
PH. 7.6
Amonia. 0
Nitrite. 0
Nitrates are at 10.
We have a fully live planted tank.
1 airstone, 1 filter(biowheel), 1 sponge filter.

No just 1 fish has it. That was just from the other side of the fish the picture in the comments.
Every other fish are fine, eating well, schooling and swimming normally.